Factor repeated headless-test setup into shared helpers

Nearly every headless UI test opened with the same four-line prologue
(resolve the shared MainWindow, show it, cast its DataContext, wait for the
assembly list), then repeated the corelib lookup, the EnsureLazyChildren +
Children.OfType<T>().Single() drill, the registry single-by-header lookups,
and the open-an-assembly-and-wait dance. The duplication made the intent of
each test hard to see and every signature tweak a suite-wide edit.

Collapse those into TestHarness (BootAsync, OpenAssemblyAsync, GetCommand,
GetEntry) and TreeNavigation extensions (FindCoreLib, GetChild<T>, Expand<T>),
then apply them across the suite. Net ~865 lines of boilerplate removed with
no change in behaviour; the full headless suite stays green.

ILSpy.Tests/Analyzers/AnalyzeContextMenuTests.cs

@@ -43,10 +43,7 @@ public class AnalyzeContextMenuTests
 	[AvaloniaTest]
 	public async Task Analyze_Entry_Is_Registered_With_The_Localised_Header()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
 		registry.Entries.Should().Contain(e => e.Metadata.Header == nameof(Resources.Analyze),
@@ -59,15 +56,10 @@ public class AnalyzeContextMenuTests
 		// AnalyzeContextMenuEntry visibility contract: visible when every selected node is
 		// an IMemberTreeNode (types, methods, fields, properties, events), hidden otherwise.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources.Analyze))
-			.Value;
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry(nameof(Resources.Analyze));
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -91,15 +83,10 @@ public class AnalyzeContextMenuTests
 		// selection. The pane's Root.Children must hold one AnalyzedTypeTreeNode wrapping
 		// the same type definition after the call.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources.Analyze))
-			.Value;
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry(nameof(Resources.Analyze));
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -121,10 +108,7 @@ public class AnalyzeContextMenuTests
 		// Ctrl+R while a member is selected on the assembly tree pane must surface the
 		// member in the analyzer pane — the same end-state as right-click + Analyze.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 
 		var pane = await window.WaitForComponent<global::ILSpy.AssemblyTree.AssemblyListPane>();
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -153,15 +137,12 @@ public class AnalyzeContextMenuTests
 		// pane's Root.Children, but if the pane is hidden the user doesn't see anything
 		// happen. Execute should call DockWorkspace.ShowToolPane so the pane surfaces.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var dockWorkspace = AppComposition.Current.GetExport<global::ILSpy.Docking.DockWorkspace>();
 		var analyzerVm = AppComposition.Current.GetExport<AnalyzerTreeViewModel>();
 		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
-			.Entries.Single(e => e.Metadata.Header == nameof(Resources.Analyze)).Value;
+			.GetEntry(nameof(Resources.Analyze));
 
 		// Pick a method that won't already be in the analyzer pane.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -208,14 +189,11 @@ public class AnalyzeContextMenuTests
 		// the leaf result rows below had perfectly good icons. Every materialised node in
 		// the analyzer tree must report a non-null Icon for the cell template to render.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var analyzerVm = AppComposition.Current.GetExport<AnalyzerTreeViewModel>();
 		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
-			.Entries.Single(e => e.Metadata.Header == nameof(Resources.Analyze)).Value;
+			.GetEntry(nameof(Resources.Analyze));
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");

ILSpy.Tests/Analyzers/AnalyzedMethodTreeNodeTests.cs

@@ -47,10 +47,7 @@ public class AnalyzedMethodTreeNodeTests
 		// in the pane rather than the bare method name. An optional `prefix` argument lets
 		// accessor / event-helper nodes prepend a tag without touching the entity string.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -74,10 +71,7 @@ public class AnalyzedMethodTreeNodeTests
 		// are "Uses" (MethodUsesAnalyzer, body scanner) and "Used By" (MethodUsedByAnalyzer,
 		// call-site scanner).
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");

ILSpy.Tests/Analyzers/AnalyzedTypeTreeNodeTests.cs

@@ -47,13 +47,9 @@ public class AnalyzedTypeTreeNodeTests
 		// the rest of the assembly tree uses — so users see "System.Object" exactly the way
 		// it appears under the assembly node, not a raw ReflectionName.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull();
 		module.Should().NotBeNull();
 		var typeSystem = module!.GetTypeSystemOrNull();
@@ -76,13 +72,9 @@ public class AnalyzedTypeTreeNodeTests
 		// non-static reference type like System.Object, "Used By", "Instantiated By", "Exposed
 		// By", and "Extension Methods" all match.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull();
 		module.Should().NotBeNull();
 		var typeSystem = module!.GetTypeSystemOrNull();

ILSpy.Tests/Analyzers/AnalyzerConstructorUsesTests.cs

@@ -49,10 +49,7 @@ public class AnalyzerConstructorUsesTests
 	[AvaloniaTest]
 	public async Task Used_By_Analyzer_Runs_Over_A_Constructor_Without_Throwing()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		// Pick any concrete type with at least one public instance ctor; the type itself
 		// doesn't matter — what matters is that the symbol fed to the analyzer is a method
@@ -93,10 +90,7 @@ public class AnalyzerConstructorUsesTests
 		// MethodUsesAnalyzer can return type/method/field/property/event ISymbols — the
 		// path most likely to surface a switch-arm hole for some result shape we missed.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Lookup`2");
@@ -122,10 +116,7 @@ public class AnalyzerConstructorUsesTests
 	[AvaloniaTest]
 	public async Task Analyze_Pushes_A_Constructor_Onto_The_Pane_Without_Throwing()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Lookup`2");

ILSpy.Tests/Analyzers/AnalyzerErrorDisplayTests.cs

@@ -75,22 +75,16 @@ public class AnalyzerErrorDisplayTests
 	[AvaloniaTest]
 	public async Task CopyErrorMessage_Entry_Is_Visible_Only_For_Error_Nodes()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources.CopyErrorMessage))
-			.Value;
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry(nameof(Resources.CopyErrorMessage));
 
 		var errorNode = new AnalyzerErrorNode(new InvalidOperationException("boom"));
 		entry.IsVisible(new TextViewContext { SelectedTreeNodes = new SharpTreeNode[] { errorNode } })
 			.Should().BeTrue("Copy Error Message must surface for AnalyzerErrorNode rows");
 
-		var unrelatedNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(
-			typeof(object).Assembly.GetName().Name!);
+		var unrelatedNode = vm.AssemblyTreeModel.FindCoreLib();
 		entry.IsVisible(new TextViewContext { SelectedTreeNodes = new SharpTreeNode[] { unrelatedNode } })
 			.Should().BeFalse("non-error rows must not see the Copy Error Message entry");
 	}

ILSpy.Tests/Analyzers/AnalyzerNavigationTests.cs

@@ -50,10 +50,7 @@ public class AnalyzerNavigationTests
 		// Double-click on an analyser result must navigate back to the entity's home in
 		// the assembly tree — same UX as clicking a hyperlink in the decompiler view.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -69,10 +66,7 @@ public class AnalyzerNavigationTests
 	[AvaloniaTest]
 	public async Task ActivateItem_On_An_AnalyzedMethodTreeNode_Selects_The_MethodTreeNode_Sibling()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");

ILSpy.Tests/Analyzers/AnalyzerPaneCopyResultsTests.cs

@@ -42,15 +42,10 @@ public class AnalyzerPaneCopyResultsTests
 	[AvaloniaTest]
 	public async Task CopyResults_Entry_Is_Visible_For_AnalyzerSearchTreeNode_And_Hidden_Otherwise()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == "Copy results")
-			.Value;
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry("Copy results");
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -71,15 +66,10 @@ public class AnalyzerPaneCopyResultsTests
 	[AvaloniaTest]
 	public async Task CopyResults_Execute_Writes_Each_Child_On_Its_Own_Line()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == "Copy results")
-			.Value;
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry("Copy results");
 
 		// Build a search node and stuff some deterministic children in directly, bypassing
 		// the analyzer altogether — the copy entry should only care about Text values.

ILSpy.Tests/Analyzers/AnalyzerPaneRemoveTests.cs

@@ -42,10 +42,7 @@ public class AnalyzerPaneRemoveTests
 	[AvaloniaTest]
 	public async Task Remove_Entry_Is_Visible_Only_For_Top_Level_Analysed_Entities()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
 		var entry = registry.Entries
@@ -68,10 +65,7 @@ public class AnalyzerPaneRemoveTests
 	[AvaloniaTest]
 	public async Task Remove_Execute_Drops_The_Selected_Root_Children()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
 		var entry = registry.Entries

ILSpy.Tests/Analyzers/AnalyzerResultHighlightTests.cs

@@ -57,10 +57,7 @@ public class AnalyzerResultHighlightTests
 		// MessageBus → AssemblyTreeModel's subscription selects the Reference's tree node
 		// AND pushes Source onto the active decompiler tab's HighlightedReference.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -94,10 +91,7 @@ public class AnalyzerResultHighlightTests
 		// so the subscriber must skip the HighlightedReference assignment rather than blow
 		// away an active highlight.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(

ILSpy.Tests/Analyzers/AnalyzerSearchTreeNodeTests.cs

@@ -50,10 +50,7 @@ public class AnalyzerSearchTreeNodeTests
 		// the elapsed-time stamp ("Used By (N in M ms)"). This single test exercises all
 		// three contracts on a small fixture so it stays fast.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		// Pick a small target so the analyzer completes quickly in headless. The "Used By"
 		// analyzer applies to every concrete type; we use a relatively unused type from
@@ -96,14 +93,10 @@ public class AnalyzerSearchTreeNodeTests
 		// a clean fetch. The "Used By" analyzer over a deeply-referenced type takes long
 		// enough that we can race a collapse against it.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		// Object is referenced by almost every type — the analyzer streams for a while.
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull();
 		module.Should().NotBeNull();
 		var typeSystem = module!.GetTypeSystemOrNull();

ILSpy.Tests/Analyzers/AnalyzerTreeNodeTests.cs

@@ -49,10 +49,7 @@ public class AnalyzerTreeNodeTests
 		// reaches the list, (b) the order is monotonically non-decreasing by Metadata.Order
 		// so users see headers in the same sequence the WPF host produces.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var analyzers = AnalyzerTreeNode.Analyzers.ToArray();
 		analyzers.Should().HaveCountGreaterThanOrEqualTo(15,
@@ -76,14 +73,10 @@ public class AnalyzerTreeNodeTests
 		// every Analyzed*TreeNode's symbol references a module the new list may not
 		// contain. The root must wipe its children rather than risk a stale navigation.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var root = new AnalyzerRootNode();
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull();
 		module.Should().NotBeNull("the test needs a loaded CoreLib module to seed a sample analyzed entry");
 

ILSpy.Tests/AssemblyList/AssemblyTreeContextMenuTests.cs

@@ -28,11 +28,8 @@ using AwesomeAssertions;
 using ICSharpCode.ILSpyX.TreeView;
 
 using ILSpy;
-using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -49,10 +46,7 @@ public class AssemblyTreeContextMenuTests
 		// without further per-entry plumbing.
 
 		// Arrange + Act — boot the window so the pane materialises.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 
 		// Assert — TreeGrid carries a ContextMenu. (The menu may be empty if no entries are
 		// registered yet; verifies only that the host is in place.)
@@ -71,10 +65,7 @@ public class AssemblyTreeContextMenuTests
 
 		// Arrange — boot, select an assembly node, install a stub entry that records the
 		// context it sees on Execute.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		vm.AssemblyTreeModel.SelectNode(assemblyNode);

ILSpy.Tests/AssemblyList/AssemblyTreeDragReorderTests.cs

@@ -29,11 +29,8 @@ using AwesomeAssertions;
 
 using ICSharpCode.ILSpyX;
 
-using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -48,10 +45,7 @@ public class AssemblyTreeDragReorderTests
 		// Mirrors WPF's SharpTreeView AllowDropOrder=True — the assembly tree must opt in to
 		// ProDataGrid's row-drag-drop machinery (otherwise the handler we wire below is never
 		// asked to validate anything).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 
@@ -71,10 +65,7 @@ public class AssemblyTreeDragReorderTests
 	{
 		// The pane owns the handler instance — it builds one from the model's AssemblyList so
 		// dropping into the grid mutates the same list that file-open and Unload mutate.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 
@@ -88,10 +79,7 @@ public class AssemblyTreeDragReorderTests
 		// the underlying AssemblyList reordered. The handler is responsible for turning
 		// HierarchicalNode wrappers (or bare AssemblyTreeNodes) into LoadedAssembly refs and
 		// calling AssemblyList.Move with the correct insert index.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 2);
+		var (window, vm) = await TestHarness.BootAsync(2);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
@@ -123,10 +111,7 @@ public class AssemblyTreeDragReorderTests
 		// "Inside" would mean dropping one assembly as a child of another — there's no such
 		// relationship in the model, so the handler must refuse it (the grid then renders the
 		// "not allowed" cursor).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 2);
+		var (window, vm) = await TestHarness.BootAsync(2);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 
@@ -143,10 +128,7 @@ public class AssemblyTreeDragReorderTests
 	{
 		// Dropping onto a child of an assembly (a namespace or type) must not reorder anything
 		// — that target doesn't live in AssemblyList at all.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 
@@ -165,10 +147,7 @@ public class AssemblyTreeDragReorderTests
 	{
 		// Sub-nodes (namespaces, types, etc.) must not be picked up by the reorder gesture —
 		// only top-level AssemblyTreeNodes are eligible source items.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 2);
+		var (window, vm) = await TestHarness.BootAsync(2);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 

ILSpy.Tests/AssemblyList/AssemblyTreeExpanderHitboxTests.cs

@@ -33,11 +33,8 @@ using AwesomeAssertions;
 
 using ICSharpCode.ILSpyX.TreeView;
 
-using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -56,14 +53,10 @@ public class AssemblyTreeExpanderHitboxTests
 		// occupy 16x16 of layout while only the 9x9 glyph receives input.
 
 		// Arrange — boot, wait for assemblies, expand a node so an expandable row is realised.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		vm.AssemblyTreeModel.SelectNode(assemblyNode);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();

ILSpy.Tests/AssemblyList/AssemblyTreeFileDropTests.cs

@@ -28,11 +28,8 @@ using Avalonia.Input;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -47,10 +44,7 @@ public class AssemblyTreeFileDropTests
 		// Avalonia's standard drag-drop pipeline routes Explorer file drops through
 		// DragDrop.AllowDrop / DragDrop.DropEvent. The grid must opt in or external drops
 		// produce a "no" cursor and never reach our handler.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
 
@@ -63,10 +57,7 @@ public class AssemblyTreeFileDropTests
 		// Simulating Windows Explorer dropping a .dll on empty space inside the tree.
 		// No target row → the new entry is appended (WPF's AssemblyListTreeNode.Drop with
 		// no target index does the same).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
 
@@ -92,10 +83,7 @@ public class AssemblyTreeFileDropTests
 	{
 		// Drop position is honoured: dragging onto the first row with Position=Before lands
 		// the opened assembly at index 0.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
 
@@ -125,10 +113,7 @@ public class AssemblyTreeFileDropTests
 	public async Task File_Drop_After_A_Target_Row_Inserts_Past_That_Index()
 	{
 		// Position=After: insertion lands at targetIndex + 1.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
 
@@ -158,10 +143,7 @@ public class AssemblyTreeFileDropTests
 		// AssemblyListTreeNode.Drop (which ends in AssemblyTreeModel.SelectNodes).
 		// Without this, the user gets no visual confirmation that the drop took, and
 		// the decompiler view stays parked on whatever was selected before.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
 
@@ -192,10 +174,7 @@ public class AssemblyTreeFileDropTests
 	{
 		// AssemblyList.OpenAssembly returns null for non-existent / non-PE paths; the
 		// handler must filter those out and never raise on the caller's behalf.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var list = vm.AssemblyTreeModel.AssemblyList!;
 

ILSpy.Tests/AssemblyList/AssemblyTreeTests.cs

@@ -48,7 +48,6 @@ using ILSpy.Metadata.CorTables;
 using ILSpy.Search;
 using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -66,22 +65,16 @@ public class AssemblyTreeTests
 		// ResourceTreeNode — both are ILSpyTreeNodes).
 
 		// Arrange — boot, wait for assemblies.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		// Act — pick CoreLib (always ships localised error-message tables), expand it.
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
+		assemblyNode.Expand();
 
 		// Assert — exactly one Resources folder, named, non-empty, every child an ILSpyTreeNode.
 		var resources = assemblyNode.Children.OfType<ResourceListTreeNode>().Single();
 		resources.Text.Should().Be(Resources._Resources);
-		resources.EnsureLazyChildren();
-		resources.IsExpanded = true;
+		resources.Expand();
 		resources.Children.Should().NotBeEmpty();
 		resources.Children.Should().AllBeAssignableTo<ILSpyTreeNode>();
 	}
@@ -93,10 +86,7 @@ public class AssemblyTreeTests
 		// Ctrl-click to multi-select rows.
 
 		// Arrange — boot, wait for assemblies + AssemblyListPane to materialise.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		// Act — locate the realised DataGrid inside the AssemblyListPane.
 		var pane = await window.WaitForComponent<AssemblyListPane>();
@@ -115,15 +105,11 @@ public class AssemblyTreeTests
 		// flip its IsSelected back without affecting siblings.
 
 		// Arrange — boot, wait for assemblies, expand Enumerable, grab two methods.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
-		typeNode.EnsureLazyChildren();
-		typeNode.IsExpanded = true;
+		typeNode.Expand();
 		var first = typeNode.Children.OfType<MethodTreeNode>()
 			.First(m => m.MethodDefinition.Name == "ElementAt");
 		var second = typeNode.Children.OfType<MethodTreeNode>()
@@ -155,10 +141,7 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies, build a fixture zip with a flat file + two
 		// folders containing one file each.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var tempDir = Path.Combine(Path.GetTempPath(), "ILSpy.Tests", System.Guid.NewGuid().ToString("N"));
 		Directory.CreateDirectory(tempDir);
@@ -174,22 +157,10 @@ public class AssemblyTreeTests
 		}
 
 		// Act — open the zip via the OpenCommand and wait for it to load.
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(zipPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, zipPath, System.StringComparison.OrdinalIgnoreCase)));
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, zipPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(zipPath);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(loaded.ShortName);
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 
 		// Assert — both nested folders show as PackageFolderTreeNodes; the flat file shows as
 		// a ResourceTreeNode with its original entry name.
@@ -212,18 +183,12 @@ public class AssemblyTreeTests
 		// string-table or inner ResourceEntryNode children (depending on payload type).
 
 		// Arrange — boot, wait for assemblies, expand CoreLib's Resources folder.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
+		assemblyNode.Expand();
 		var resources = assemblyNode.Children.OfType<ResourceListTreeNode>().Single();
-		resources.EnsureLazyChildren();
-		resources.IsExpanded = true;
+		resources.Expand();
 
 		// Act — look up the first .resources file and select it.
 		var resourceFileNode = resources.Children.OfType<ResourcesFileTreeNode>().FirstOrDefault();
@@ -246,14 +211,10 @@ public class AssemblyTreeTests
 		// AssemblyReferenceTreeNodes — one per AssemblyRef row.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 
 		// Act — find the References folder.
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
@@ -261,8 +222,7 @@ public class AssemblyTreeTests
 		// Assert — folder is named, expands to non-empty AssemblyReferenceTreeNode children.
 		refFolder.Text.Should().Be(Resources.References);
 
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 		refFolder.Children.Should().NotBeEmpty();
 		refFolder.Children.Should().AllBeAssignableTo<AssemblyReferenceTreeNode>();
 	}
@@ -277,14 +237,10 @@ public class AssemblyTreeTests
 		// Arrange — boot, wait for assemblies, force lazy children on assembly + References.
 		// Stay model-only (no IsExpanded = true) — visually expanding rows would render the
 		// icons, triggering the resolver and losing the lazy "loading" state we're asserting.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
+		var refFolder = assemblyNode.GetChild<ReferenceFolderTreeNode>();
 		refFolder.EnsureLazyChildren();
 
 		var refNode = refFolder.Children.OfType<AssemblyReferenceTreeNode>()
@@ -309,27 +265,20 @@ public class AssemblyTreeTests
 		// TypeRef rows the host assembly imports from that reference.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq → References → System.Runtime.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 
 		var refNode = refFolder.Children.OfType<AssemblyReferenceTreeNode>()
 			.First(n => n.AssemblyReference.Name == "System.Runtime");
-		refNode.EnsureLazyChildren();
-		refNode.IsExpanded = true;
+		refNode.Expand();
 
 		// Act — find the ReferencedTypes subnode and expand it.
 		var typesNode = refNode.Children.OfType<AssemblyReferenceReferencedTypesTreeNode>().Single();
-		typesNode.EnsureLazyChildren();
-		typesNode.IsExpanded = true;
+		typesNode.Expand();
 
 		// Assert — header text contains the expected label, and the children include at least
 		// one TypeReferenceTreeNode.
@@ -345,14 +294,10 @@ public class AssemblyTreeTests
 		// search to land on a specific namespace.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 
 		// Act — look up the System.Linq namespace.
 		var ns = assemblyNode.FindNamespaceNode("System.Linq");
@@ -370,14 +315,10 @@ public class AssemblyTreeTests
 		// to land on a specific type by ITypeDefinition.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 
 		// Act — resolve System.Linq.Enumerable's ITypeDefinition, then look up its tree node.
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull()!;
@@ -399,17 +340,12 @@ public class AssemblyTreeTests
 		// top of the underlying Method/Field glyph — verifies that overlay routing works.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq → References.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 
 		// Act — walk every reference's ReferencedTypes subtree until we find a TypeRef with
 		// at least one MemberReference. System.Linq imports plenty (delegates, Func ctors)
@@ -417,17 +353,14 @@ public class AssemblyTreeTests
 		MemberReferenceTreeNode? memberRefNode = null;
 		foreach (var refNode in refFolder.Children.OfType<AssemblyReferenceTreeNode>())
 		{
-			refNode.EnsureLazyChildren();
-			refNode.IsExpanded = true;
+			refNode.Expand();
 			var typesNode = refNode.Children.OfType<AssemblyReferenceReferencedTypesTreeNode>().FirstOrDefault();
 			if (typesNode == null)
 				continue;
-			typesNode.EnsureLazyChildren();
-			typesNode.IsExpanded = true;
+			typesNode.Expand();
 			foreach (var typeRef in typesNode.Children.OfType<TypeReferenceTreeNode>())
 			{
-				typeRef.EnsureLazyChildren();
-				typeRef.IsExpanded = true;
+				typeRef.Expand();
 				memberRefNode = typeRef.Children.OfType<MemberReferenceTreeNode>().FirstOrDefault();
 				if (memberRefNode != null)
 					break;
@@ -452,10 +385,7 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies. Open mscorlib explicitly: it's the canonical
 		// type-forwarder facade and we need a known forwarder regardless of default startup.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibDir = System.IO.Path.GetDirectoryName(typeof(object).Assembly.Location)!;
 		var mscorlibPath = System.IO.Path.Combine(coreLibDir, "mscorlib.dll");
@@ -464,37 +394,22 @@ public class AssemblyTreeTests
 			Assert.Inconclusive($"mscorlib.dll not present next to System.Private.CoreLib at {coreLibDir} — runtime layout differs.");
 			return;
 		}
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(mscorlibPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, mscorlibPath, System.StringComparison.OrdinalIgnoreCase)));
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, mscorlibPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(mscorlibPath);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(loaded.ShortName);
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 
 		// Act — walk every reference's ReferencedTypes for an ExportedTypeTreeNode.
 		ExportedTypeTreeNode? exportedNode = null;
 		foreach (var refNode in refFolder.Children.OfType<AssemblyReferenceTreeNode>())
 		{
-			refNode.EnsureLazyChildren();
-			refNode.IsExpanded = true;
+			refNode.Expand();
 			var typesNode = refNode.Children.OfType<AssemblyReferenceReferencedTypesTreeNode>().FirstOrDefault();
 			if (typesNode == null)
 				continue;
-			typesNode.EnsureLazyChildren();
-			typesNode.IsExpanded = true;
+			typesNode.Expand();
 			exportedNode = typesNode.Children.OfType<ExportedTypeTreeNode>().FirstOrDefault();
 			if (exportedNode != null)
 				break;
@@ -514,25 +429,19 @@ public class AssemblyTreeTests
 		// — building a transitive reference tree the user can drill into.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq → References.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 
 		// Act — many BCL refs resolve to facade assemblies with zero transitive references;
 		// iterate until we find one that has its own AssemblyRefs.
 		bool foundTransitive = false;
 		foreach (var refNode in refFolder.Children.OfType<AssemblyReferenceTreeNode>())
 		{
-			refNode.EnsureLazyChildren();
-			refNode.IsExpanded = true;
+			refNode.Expand();
 			if (refNode.Children.OfType<AssemblyReferenceTreeNode>().Any())
 			{
 				foundTransitive = true;
@@ -551,17 +460,12 @@ public class AssemblyTreeTests
 		// assembly's AssemblyTreeNode in the tree — auto-loading it if necessary.
 
 		// Arrange — boot, wait for assemblies, expand System.Linq → References.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 		var refFolder = assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single();
-		refFolder.EnsureLazyChildren();
-		refFolder.IsExpanded = true;
+		refFolder.Expand();
 
 		var refNode = refFolder.Children.OfType<AssemblyReferenceTreeNode>()
 			.Single(n => n.AssemblyReference.Name == "System.Runtime");
@@ -591,25 +495,10 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies, open System.Net.Http (its CancelPendingRequests
 		// pulls in plenty of currently-unreferenced types).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var newAsmPath = typeof(System.Net.Http.HttpClient).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(newAsmPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase)));
-
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(newAsmPath);
 
 		var initialFiles = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
 			.Select(a => a.FileName)
@@ -619,8 +508,7 @@ public class AssemblyTreeTests
 		// assemblies to appear.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			loaded.ShortName, "System.Net.Http", "System.Net.Http.HttpClient");
-		typeNode.EnsureLazyChildren();
-		typeNode.IsExpanded = true;
+		typeNode.Expand();
 		var methodNode = typeNode.Children.OfType<MethodTreeNode>()
 			.First(m => m.MethodDefinition.Name == "CancelPendingRequests");
 
@@ -653,30 +541,14 @@ public class AssemblyTreeTests
 		// user actually opened.
 
 		// Arrange — boot, wait for assemblies, open + select a method that triggers auto-load.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var newAsmPath = typeof(System.Net.Http.HttpClient).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(newAsmPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase)));
-
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(newAsmPath);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			loaded.ShortName, "System.Net.Http", "System.Net.Http.HttpClient");
-		typeNode.EnsureLazyChildren();
-		typeNode.IsExpanded = true;
+		typeNode.Expand();
 		var methodNode = typeNode.Children.OfType<MethodTreeNode>()
 			.First(m => m.MethodDefinition.Name == "CancelPendingRequests");
 
@@ -717,28 +589,13 @@ public class AssemblyTreeTests
 		// matching tree node, and select that node so the user sees their newly-opened file.
 
 		// Arrange — boot, wait for assemblies, snapshot the initial count.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var newAsmPath = typeof(System.Net.Http.HttpClient).Assembly.Location;
 		var initialCount = vm.AssemblyTreeModel.AssemblyList!.Count;
 
 		// Act — fire OpenCommand on a new path and wait for the load to complete.
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(newAsmPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase)));
-
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, newAsmPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(newAsmPath);
 
 		// Assert — count incremented by one; the corresponding tree node points at the
 		// freshly-loaded assembly and is selected.
@@ -758,24 +615,18 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies, expand a sub-tree large enough to scroll. Park
 		// the viewport mid-list so clicks are at non-zero offset.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var enumerable = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
-		enumerable.EnsureLazyChildren();
-		enumerable.IsExpanded = true;
+		enumerable.Expand();
 		var ns = (NamespaceTreeNode)enumerable.Parent!;
-		ns.EnsureLazyChildren();
-		ns.IsExpanded = true;
+		ns.Expand();
 		var asm = (AssemblyTreeNode)ns.Parent!;
 		asm.IsExpanded = true;
 		foreach (var child in ns.Children.OfType<TypeTreeNode>())
 		{
-			child.EnsureLazyChildren();
-			child.IsExpanded = true;
+			child.Expand();
 		}
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
@@ -838,19 +689,13 @@ public class AssemblyTreeTests
 		// method — that's how nodes (e.g. resources) opt into a custom save dialog/format.
 
 		// Arrange — boot, wait for assemblies, plant a probe node that records when Save runs.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var probe = new SaveProbeNode();
 		vm.AssemblyTreeModel.SelectNode(probe);
 
 		// Act — fire the Save Code main-menu command.
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var saveCmd = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._SaveCode))
-			.CreateExport().Value;
+		var saveCmd = AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(nameof(Resources._SaveCode));
 		saveCmd.Execute(null);
 
 		// Assert — probe's Save() ran.
@@ -877,15 +722,11 @@ public class AssemblyTreeTests
 		// write the resulting C# to the user-picked path.
 
 		// Arrange — boot, wait for assemblies, select a method, wait for the in-tab decompile.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
-		typeNode.EnsureLazyChildren();
-		typeNode.IsExpanded = true;
+		typeNode.Expand();
 		var method = typeNode.Children.OfType<MethodTreeNode>()
 			.First(m => m.MethodDefinition.Name == "AsEnumerable");
 		vm.AssemblyTreeModel.SelectNode(method);
@@ -929,10 +770,7 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies, pick the assembly to evict + a sibling we
 		// expect to survive, select the sacrificial one, focus the grid.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var sacrificialName = typeof(System.Linq.Enumerable).Assembly.GetName().Name!;
 		var survivorName = typeof(object).Assembly.GetName().Name!;
@@ -980,17 +818,11 @@ public class AssemblyTreeTests
 		// The Clear Assembly List menu command must drop every entry from the active list.
 
 		// Arrange — boot, wait for assemblies, baseline the non-empty count.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 		vm.AssemblyTreeModel.AssemblyList!.Count.Should().BeGreaterThan(0);
 
 		// Act — fire the Clear command.
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var clearCmd = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources.ClearAssemblyList))
-			.CreateExport().Value;
+		var clearCmd = AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(nameof(Resources.ClearAssemblyList));
 		clearCmd.CanExecute(null).Should().BeTrue("non-empty list must enable Clear");
 		clearCmd.Execute(null);
 
@@ -1008,10 +840,7 @@ public class AssemblyTreeTests
 		// Arrange — boot, wait for assemblies, plant a "broken" entry: a real file that isn't
 		// a PE binary. AssemblyList records it but the load fails — that's the state the
 		// command targets.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var brokenPath = System.IO.Path.Combine(
 			System.IO.Path.GetTempPath(),
@@ -1035,10 +864,7 @@ public class AssemblyTreeTests
 			validBefore.Should().NotBeEmpty("at least one real assembly must remain to verify it's spared");
 
 			// Act — fire the command.
-			var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-			var cmd = registry.Commands
-				.Single(c => c.Metadata.Header == nameof(Resources._RemoveAssembliesWithLoadErrors))
-				.CreateExport().Value;
+			var cmd = AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(nameof(Resources._RemoveAssembliesWithLoadErrors));
 			cmd.CanExecute(null).Should().BeTrue("a broken entry must enable the command");
 			cmd.Execute(null);
 
@@ -1069,10 +895,7 @@ public class AssemblyTreeTests
 
 		// Arrange — boot, wait for assemblies, locate the realised DataGrid and the
 		// HierarchicalModel wrapper for the assembly node.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
@@ -1084,8 +907,7 @@ public class AssemblyTreeTests
 		hNode!.IsExpanded.Should().BeFalse("baseline: top-level assembly rows start collapsed");
 
 		// Act — production-shaped action: just toggle the model property.
-		assemblyNode.EnsureLazyChildren();
-		assemblyNode.IsExpanded = true;
+		assemblyNode.Expand();
 
 		// Drain the dispatcher so the wiring (PropertyChanged → hm.Expand) settles.
 		for (int i = 0; i < 5; i++)
@@ -1106,10 +928,7 @@ public class AssemblyTreeTests
 		// tab opens with the supplied node decompiled, the existing tab keeps its content,
 		// and the assembly-tree selection is pulled across to the new tab's source node
 		// (the active tab and the tree are kept in lockstep).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -1147,10 +966,7 @@ public class AssemblyTreeTests
 	{
 		// Activate two tabs (each carrying a different SourceNode), then flip the active
 		// dockable back to the first one and confirm the assembly-tree selection follows.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -1188,19 +1004,12 @@ public class AssemblyTreeTests
 		// DecompilerTabPageModel. Exercises the regression behind "there can't be multiple
 		// metadata views": before the OpenNodeInNewTab consolidation, the new-tab path
 		// always built a decompiler tab and the metadata page-type couldn't be carved out.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var tablesNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
+		var typeDefNode = tablesNode.GetChild<TypeDefTableTreeNode>();
 
 		// First metadata view via tree-selection (active tab gets MetadataTablePageModel content).
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
@@ -1229,10 +1038,7 @@ public class AssemblyTreeTests
 		// ILSpy. Without this, tree-based navigation can't reach get_X / set_X — and
 		// MMB on a property-accessor row in the metadata grid silently no-ops because
 		// FindTreeNode can't resolve the IMethod to a tree node.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringTypeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1257,10 +1063,7 @@ public class AssemblyTreeTests
 		// Events surface add_X / remove_X (and invoke_X if present) as MethodTreeNode
 		// children, mirroring WPF ILSpy. Same regression-protection as the property
 		// case above.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		// AppDomain.AssemblyLoad is a public, well-known event in CoreLib.
@@ -1285,10 +1088,7 @@ public class AssemblyTreeTests
 		// the per-node Filter must hide them under the default ShowApiLevel — they're
 		// rendered as part of the parent property in the C# surface, so the tree should
 		// not duplicate them as standalone rows. ShowAll is the explicit opt-in.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringTypeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1317,10 +1117,7 @@ public class AssemblyTreeTests
 		// an IMethod whose AccessorOwner is the IProperty. Without the AccessorOwner-aware
 		// branch in FindMemberNode, the lookup falls through to "MethodTreeNode child of the
 		// type" and returns null because accessors live under the property, not the type.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringTypeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1346,10 +1143,7 @@ public class AssemblyTreeTests
 		// expect a single BaseTypesEntryNode whose label ends in "Object".
 
 		// Arrange — boot, wait for assemblies, drill into System.Exception.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1371,10 +1165,7 @@ public class AssemblyTreeTests
 		// Activating a BaseTypesEntryNode jumps the assembly-tree selection to the underlying
 		// type node — same gesture the WPF tree uses to walk an inheritance chain.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1404,10 +1195,7 @@ public class AssemblyTreeTests
 		// scan the assembly list synchronously — for the small fixture loaded in tests this
 		// yields a few hits per common base (e.g. SystemException is derived from Exception).
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1425,10 +1213,7 @@ public class AssemblyTreeTests
 	{
 		// Sealed types can't be derived from, so the DerivedTypes sub-tree must not appear.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1444,10 +1229,7 @@ public class AssemblyTreeTests
 	{
 		// System.Object has no base types, so the BaseTypes sub-tree must not appear.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var objectNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -1498,10 +1280,7 @@ public class AssemblyTreeTests
 		// must NOT return Hidden purely because the field's name doesn't contain the active
 		// SearchTerm — only ShowApiLevel + ShowMember remain valid hiding criteria.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var settings = AppComposition.Current.GetExport<SettingsService>()
 			.SessionSettings.LanguageSettings;
@@ -1533,13 +1312,9 @@ public class AssemblyTreeTests
 		// NOT as bare TypeTreeNodes hanging directly off the assembly node.
 
 		// Arrange — boot, wait for assemblies, locate CoreLib.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		assemblyNode.EnsureLazyChildren();
 
 		// Assert — exactly one global-namespace node, labelled "-", non-empty contents.
@@ -1570,10 +1345,7 @@ public class AssemblyTreeTests
 		// the current cell to the last row (the user reported having to press it twice).
 
 		// Arrange — boot, wait for assemblies, realise the grid.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
@@ -1619,10 +1391,7 @@ public class AssemblyTreeTests
 		// one row must collapse the selection down to just that row.
 
 		// Arrange — boot, wait for assemblies, realise the grid, select everything.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = await pane.WaitForComponent<DataGrid>();
@@ -1670,16 +1439,10 @@ public class AssemblyTreeTests
 		// to do nothing.
 
 		// Arrange -- boot, open System.Net.Http (it references many assemblies not in the list).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var newAsmPath = typeof(System.Net.Http.HttpClient).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
+		var openCommand = AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(nameof(Resources._Open));
 		openCommand.Execute(newAsmPath);
 		await Waiters.WaitForAsync(() =>
 			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>

ILSpy.Tests/AssemblyList/ReloadAssemblyContextMenuTests.cs

@@ -32,7 +32,6 @@ using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
 using ILSpy.Views;
 
 using NUnit.Framework;
@@ -68,14 +67,9 @@ public class ReloadAssemblyContextMenuTests
 		// pollute their right-click menus.
 
 		// Arrange — boot, locate the registered entry.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var reloadEntry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources._Reload))
-			.Value;
+		var reloadEntry = registry.GetEntry(nameof(Resources._Reload));
 
 		var asm = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -106,10 +100,7 @@ public class ReloadAssemblyContextMenuTests
 		// the same file name.
 
 		// Arrange — boot, snapshot the LoadedAssembly we'll reload.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
 

ILSpy.Tests/AssemblyList/RemoveAssemblyContextMenuTests.cs

@@ -33,7 +33,6 @@ using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
 using ILSpy.Views;
 
 using NUnit.Framework;
@@ -69,14 +68,9 @@ public class RemoveAssemblyContextMenuTests
 		// e.g. method/type tree contexts.
 
 		// Arrange — boot, locate the registered entry.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var removeEntry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources._Remove))
-			.Value;
+		var removeEntry = registry.GetEntry(nameof(Resources._Remove));
 
 		var asm = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -110,10 +104,7 @@ public class RemoveAssemblyContextMenuTests
 	public async Task Right_Click_Then_Remove_On_An_Assembly_Unloads_It_From_The_List()
 	{
 		// Arrange — boot, snapshot a sacrificial assembly + a survivor.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
 
@@ -152,10 +143,7 @@ public class RemoveAssemblyContextMenuTests
 		// when right-clicking on a selected assembly node.
 
 		// Arrange — boot, select an assembly node.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		await Waiters.WaitForAsync(
 			() => window.GetVisualDescendants().OfType<AssemblyListPane>().Any());
 		var pane = await window.WaitForComponent<AssemblyListPane>();

ILSpy.Tests/AssemblyList/ShowMetadataTokensTests.cs

@@ -27,8 +27,6 @@ using AwesomeAssertions;
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -46,10 +44,7 @@ public class ShowMetadataTokensTests
 		// empty otherwise.
 
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		// Drill into a known type and grab a method node.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(

ILSpy.Tests/AssemblyList/TreeViewSettingsLiveReactivityTests.cs

@@ -29,8 +29,6 @@ using ICSharpCode.ILSpyX.TreeView;
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -48,10 +46,7 @@ public class TreeViewSettingsLiveReactivityTests
 		// 'Text' fires when the setting flips.
 
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesGridVerification.cs

@@ -27,8 +27,6 @@ using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -49,10 +47,7 @@ public class UseNestedNamespaceNodesGridVerification
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.UseNestedNamespaceNodes = false;
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
 		var grid = pane.FindControl<DataGrid>("TreeGrid")!;
 

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesLiveTests.cs

@@ -26,8 +26,6 @@ using AwesomeAssertions;
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -45,10 +43,7 @@ public class UseNestedNamespaceNodesLiveTests
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.UseNestedNamespaceNodes = false;
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		try
 		{

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesScreenshot.cs

@@ -26,8 +26,6 @@ using Avalonia.Threading;
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -43,10 +41,7 @@ public class UseNestedNamespaceNodesScreenshot
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.UseNestedNamespaceNodes = false;
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		assemblyNode.IsExpanded = true;

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesTests.cs

@@ -26,8 +26,6 @@ using AwesomeAssertions;
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -44,10 +42,7 @@ public class UseNestedNamespaceNodesTests
 		// "System.IO" siblings the default flat mode produces.
 
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		try
 		{
@@ -88,10 +83,7 @@ public class UseNestedNamespaceNodesTests
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.UseNestedNamespaceNodes = false;
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		assemblyNode.Children.Clear();

ILSpy.Tests/Docking/LayoutPersistenceTests.cs

@@ -30,7 +30,6 @@ using Dock.Model.Core;
 using ILSpy.AppEnv;
 using ILSpy.Docking;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -59,10 +58,7 @@ public class LayoutPersistenceTests
 	[AvaloniaTest]
 	public async Task SaveLayout_Followed_By_LoadLayout_Round_Trips_The_Live_Layout()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		_ = await TestHarness.BootAsync();
 
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();
 		var path = Path.Combine(Path.GetTempPath(), $"ILSpy.Layout.test.{System.Guid.NewGuid():N}.json");
@@ -127,10 +123,7 @@ public class LayoutPersistenceTests
 		// `if (factory.MainTab is not { } main) return;` and the user sees an empty
 		// editor when they click a tree node.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		_ = await TestHarness.BootAsync();
 
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();
 		var path = Path.Combine(Path.GetTempPath(), $"ILSpy.Layout.test.{System.Guid.NewGuid():N}.json");
@@ -184,10 +177,7 @@ public class LayoutPersistenceTests
 		// DocumentDock comes back with a single fresh MainTab; user's tree selection
 		// re-projects through ShowSelectedNode as on first launch.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		_ = await TestHarness.BootAsync();
 
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();
 

ILSpy.Tests/Docking/PreviewTabPromotionTests.cs

@@ -58,10 +58,7 @@ public class PreviewTabPromotionTests
 	{
 		// Open-in-new-tab tabs are explicit user intent — they should never be replaced
 		// by subsequent tree-node selections, so they're born pinned (IsPreview=false).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -81,10 +78,7 @@ public class PreviewTabPromotionTests
 		// preview tab spawns at pin time. A fresh preview tab opens lazily later, when a
 		// tree-selection change finds the active tab frozen — see
 		// Selecting_A_Different_Node_After_Pin_Opens_A_New_Preview_Tab below.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var factory = (ILSpyDockFactory)vm.DockWorkspace.Factory;
 		var previousMainTab = factory.MainTab!;
@@ -124,10 +118,7 @@ public class PreviewTabPromotionTests
 		// Pin holds the current tab's content. Selecting a different tree node while
 		// the (now-pinned) tab is active must spawn a fresh preview tab beside it and
 		// route the new content there — never overwrite the pinned tab.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var factory = (ILSpyDockFactory)vm.DockWorkspace.Factory;
 
@@ -175,10 +166,7 @@ public class PreviewTabPromotionTests
 		// Visual contract: the DocumentTabStripItem for the preview MainTab binds its
 		// FontStyle to ContentTabPage.IsPreview via BoolToFontStyleConverter.Italic.
 		// Pinned tabs and tool-pane tabs fall through to FontStyle.Normal.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 
 		// Wait for the tab strip to realise its items.
 		await Waiters.WaitForAsync(() => window.GetVisualDescendants().OfType<DocumentTabStripItem>().Any(),
@@ -223,10 +211,7 @@ public class PreviewTabPromotionTests
 		// tab so both MainTab and carve-out exist; close button becomes visible; verify
 		// the pin's right edge stays inside the tab's right edge.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		// Open a carve-out so close buttons become visible on both tabs.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -297,10 +282,7 @@ public class PreviewTabPromotionTests
 		// width if the title isn't capped. The pin button must still fit inside the tab's
 		// right edge regardless of title length.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		// Open carve-out so close button is visible.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -365,10 +347,7 @@ public class PreviewTabPromotionTests
 	[AvaloniaTest]
 	public async Task Inline_Pin_Button_Appears_On_Preview_Tab_And_Pins_When_Clicked()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 
 		await Waiters.WaitForAsync(() => window.GetVisualDescendants().OfType<DocumentTabStripItem>().Any(),
 			System.TimeSpan.FromSeconds(10));
@@ -409,10 +388,7 @@ public class PreviewTabPromotionTests
 	{
 		// The right-click context-menu Pin entry must be visible on the preview MainTab
 		// and hidden (its IsVisible flips) on pinned carve-out tabs.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
 		vm.DockWorkspace.OpenNodeInNewTab(typeNode);
@@ -442,10 +418,7 @@ public class PreviewTabPromotionTests
 		// because the App.axaml setter used a static `FontStyle="Italic"` value instead
 		// of the binding to IsPreview. A carve-out tab (IsPreview=false) must render
 		// Normal, even when sitting next to a preview MainTab.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		// Open a carve-out tab via the production code path.
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
@@ -497,10 +470,7 @@ public class PreviewTabPromotionTests
 		// (Open / Save / Remove / Exit groups in the File menu, ...). With the menu now living
 		// as a NativeMenu, separator rendering is the platform's job; this test pins only
 		// the structural claim that separators are inserted between groups.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, _) = await TestHarness.BootAsync();
 
 		var nativeMenu = global::Avalonia.Controls.NativeMenu.GetMenu(window)
 			?? throw new System.InvalidOperationException("MainMenu.Attach should have set NativeMenu on the window");
@@ -519,10 +489,7 @@ public class PreviewTabPromotionTests
 		// hover background. Both are plain Avalonia.Controls.Button instances; the close
 		// button gets its visual identity from a ControlTheme applied by Dock's tab
 		// template. Pin should copy that Theme rather than carry its own custom style.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		// Open a carve-out so the close button is visible alongside the pin (single-tab
 		// scenario hides the close button).
@@ -561,10 +528,7 @@ public class PreviewTabPromotionTests
 		// selections while any of those are active must spawn a fresh preview tab and
 		// route the new content there. This test uses a carve-out as a stand-in for the
 		// frozen-tab family — the IsWritablePreview check returns null for all three.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var factory = (ILSpyDockFactory)vm.DockWorkspace.Factory;
 

ILSpy.Tests/Docking/SingletonDocumentTabTests.cs

@@ -45,8 +45,7 @@ public class SingletonDocumentTabTests
 {
 	static void Invoke(MainWindow window, string header)
 	{
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		registry.Commands.Single(c => c.Metadata.Header == header).CreateExport().Value.Execute(null);
+		AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(header).Execute(null);
 		Avalonia.Threading.Dispatcher.UIThread.RunJobs();
 	}
 

ILSpy.Tests/MainWindow/MainToolBarLayoutTests.cs

@@ -32,7 +32,6 @@ using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.Commands;
 using ILSpy.Search;
-using ILSpy.ViewModels;
 using ILSpy.Views;
 
 using NUnit.Framework;
@@ -92,10 +91,7 @@ public class MainToolBarLayoutTests
 		// Avalonia port only had the key-binding wiring — this test pins the toolbar surface area
 		// and verifies executing the button activates the search pane.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, _) = await TestHarness.BootAsync();
 
 		var toolbar = await window.WaitForComponent<MainToolBar>();
 		var rootPanel = toolbar.GetVisualDescendants().OfType<StackPanel>()
@@ -160,10 +156,7 @@ public class MainToolBarLayoutTests
 		// the assembly-list combo each live inside a Grid wrapper (axaml lines 223 / 232 / 245),
 		// so we tag those positions by descending into the Grid for the ComboBox name.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, _) = await TestHarness.BootAsync();
 
 		var toolbar = await window.WaitForComponent<MainToolBar>();
 		var rootPanel = toolbar.GetVisualDescendants().OfType<StackPanel>()

ILSpy.Tests/MainWindow/MainWindowTests.cs

@@ -137,10 +137,7 @@ public class MainWindowTests
 		// transition to Indeterminate, and once done it must drop back to None.
 
 		// Arrange — boot window, wait for assemblies, subscribe to TaskbarProgressService events.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var service = AppComposition.Current.GetExport<TaskbarProgressService>();
 		var states = new List<TaskbarProgressState>();

ILSpy.Tests/MainWindow/WindowMenuOpenDocumentsTests.cs

@@ -27,11 +27,9 @@ using AwesomeAssertions;
 
 using ICSharpCode.ILSpy.Properties;
 
-using ILSpy.AppEnv;
 using ILSpy.Docking;
 using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -49,10 +47,7 @@ public class WindowMenuOpenDocumentsTests
 		// (NativeMenuBar's visual presenter renders these on Windows / Linux, and the
 		// system menu bar projects them on macOS).
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var firstAsm = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		vm.AssemblyTreeModel.SelectNode(firstAsm);

ILSpy.Tests/Metadata/DebugDirectoryChildNodesTests.cs

@@ -23,11 +23,7 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -96,37 +92,26 @@ public class DebugDirectoryChildNodesTests
 
 	static async Task<DebugDirectoryTreeNode> GetDebugDirectoryForCoreLib()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var debugDir = metadataNode.Children.OfType<DebugDirectoryTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var debugDir = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DebugDirectoryTreeNode>();
 		debugDir.EnsureLazyChildren();
 		return debugDir;
 	}
 
 	static async Task<(DebugDirectoryTreeNode node, int rawCount)> GetDebugDirectoryAndRawCountForCoreLib()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = (ICSharpCode.Decompiler.Metadata.PEFile)assemblyNode.LoadedAssembly.GetMetadataFileOrNull()!;
 		int rawCount = module.Reader.ReadDebugDirectory().Length;
 
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var debugDir = metadataNode.Children.OfType<DebugDirectoryTreeNode>().Single();
+		var debugDir = assemblyNode
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DebugDirectoryTreeNode>();
 		debugDir.EnsureLazyChildren();
 		return (debugDir, rawCount);
 	}

ILSpy.Tests/Metadata/EmbeddedPdbTreeTests.cs

@@ -25,13 +25,9 @@ using AwesomeAssertions;
 
 using ICSharpCode.ILSpy.Properties;
 
-using ILSpy.AppEnv;
-using ILSpy.Commands;
 using ILSpy.Metadata;
 using ILSpy.Metadata.DebugTables;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -51,38 +47,21 @@ public class EmbeddedPdbTreeTests
 		// own Tables subtree contains the PDB-only tables (Document and friends) — the
 		// canonical entry point for browsing debug metadata in the live app.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		// ICSharpCode.Decompiler ships with an embedded portable PDB (no separate .pdb in
 		// the build output) — use it as the fixture rather than the test DLL itself, which
 		// the test SDK builds with portable + sidecar PDB.
 		var testDllPath = typeof(ICSharpCode.Decompiler.Metadata.MetadataFile).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(testDllPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase)));
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(testDllPath);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(loaded.ShortName);
-		assemblyNode.EnsureLazyChildren();
 		// AssemblyTreeNode now surfaces two MetadataTreeNode children for assemblies with an
 		// embedded PDB (host metadata + a top-level PDB sibling). This test cares about the
 		// nested-under-DebugDirectory path, so pick the host metadata explicitly by label.
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>()
-			.Single(n => (string?)n.Text == Resources.Metadata);
-		metadataNode.EnsureLazyChildren();
+		var metadataNode = assemblyNode.GetChild<MetadataTreeNode>(n => (string?)n.Text == Resources.Metadata);
 
-		var debugDirectoryNode = metadataNode.Children.OfType<DebugDirectoryTreeNode>().Single();
+		var debugDirectoryNode = metadataNode.GetChild<DebugDirectoryTreeNode>();
 		debugDirectoryNode.EnsureLazyChildren();
 
 		// The embedded-PDB sub-tree is a MetadataTreeNode whose own children include a
@@ -92,8 +71,7 @@ public class EmbeddedPdbTreeTests
 			"the test DLL ships with an embedded portable PDB, so the Debug Directory must expose its metadata as a nested MetadataTreeNode");
 		embeddedPdbNode!.Text.Should().Be("Debug Metadata (Embedded)");
 
-		embeddedPdbNode.EnsureLazyChildren();
-		var pdbTables = embeddedPdbNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var pdbTables = embeddedPdbNode.GetChild<MetadataTablesTreeNode>();
 		pdbTables.EnsureLazyChildren();
 		pdbTables.Children.OfType<DocumentTableTreeNode>().Should().NotBeEmpty(
 			"the embedded PDB's Tables subtree must expose the Document table — the lead debug-only table that always has rows in a populated PDB");
@@ -107,24 +85,10 @@ public class EmbeddedPdbTreeTests
 		// folder), not only nested under DebugDirectory. Without this, users have to drill
 		// three levels deep to reach PDB metadata — same content, harder to find.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var testDllPath = typeof(ICSharpCode.Decompiler.Metadata.MetadataFile).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(testDllPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase)));
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(testDllPath);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(loaded.ShortName);
 		assemblyNode.EnsureLazyChildren();

ILSpy.Tests/Metadata/HeapTreeTests.cs

@@ -23,11 +23,7 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -45,15 +41,9 @@ public class HeapTreeTests
 		// reachable from the Metadata folder regardless of whether the file's debug
 		// metadata is present.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 		metadataNode.EnsureLazyChildren();
 
 		metadataNode.Children.OfType<StringHeapTreeNode>().Should().ContainSingle();
@@ -69,17 +59,11 @@ public class HeapTreeTests
 		// row through Avalonia's built-in row virtualisation, so unlike the Phase 1 text
 		// dump there's no preview cap and no truncation footer.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var heapNode = metadataNode.Children.OfType<StringHeapTreeNode>().Single();
+		var heapNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<StringHeapTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(heapNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -93,17 +77,11 @@ public class HeapTreeTests
 	[AvaloniaTest]
 	public async Task GuidHeapTreeNode_Opens_Grid_Tab_With_Index_Length_Value_Columns()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var heapNode = metadataNode.Children.OfType<GuidHeapTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var heapNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<GuidHeapTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(heapNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -118,17 +96,11 @@ public class HeapTreeTests
 	[AvaloniaTest]
 	public async Task BlobHeapTreeNode_Opens_Grid_Tab_With_Offset_Length_Value_Columns()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var heapNode = metadataNode.Children.OfType<BlobHeapTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var heapNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<BlobHeapTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(heapNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();

ILSpy.Tests/Metadata/HideEmptyMetadataTablesTests.cs

@@ -29,7 +29,6 @@ using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -47,10 +46,7 @@ public class HideEmptyMetadataTablesTests
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.HideEmptyMetadataTables = true;
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var tables = LoadTablesNode(vm, coreLibName, out var metadata);
@@ -71,10 +67,7 @@ public class HideEmptyMetadataTablesTests
 		settings.HideEmptyMetadataTables = false;
 		try
 		{
-			var window = AppComposition.Current.GetExport<MainWindow>();
-			window.Show();
-			var vm = (MainWindowViewModel)window.DataContext!;
-			await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+			var (_, vm) = await TestHarness.BootAsync();
 
 			var coreLibName = typeof(object).Assembly.GetName().Name!;
 			var tables = LoadTablesNode(vm, coreLibName, out var _);
@@ -94,10 +87,9 @@ public class HideEmptyMetadataTablesTests
 		out System.Reflection.Metadata.MetadataReader metadata)
 	{
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(assemblyName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tables = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var tables = assemblyNode
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
 		tables.EnsureLazyChildren();
 		metadata = assemblyNode.LoadedAssembly.GetMetadataFileOrNull()!.Metadata;
 		return tables;

ILSpy.Tests/Metadata/HideEmptyMetadataTablesUiBindingTests.cs

@@ -28,7 +28,6 @@ using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -46,10 +45,7 @@ public class HideEmptyMetadataTablesUiBindingTests
 		// SessionSettings.HideEmptyMetadataTables (an orphaned parallel setting).
 
 		var settings = AppComposition.Current.GetExport<SettingsService>();
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 
@@ -80,10 +76,9 @@ public class HideEmptyMetadataTablesUiBindingTests
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(assemblyName);
 		assemblyNode.Children.Clear();
 		assemblyNode.LazyLoading = true;
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tables = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var tables = assemblyNode
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
 		tables.EnsureLazyChildren();
 		return tables.Children.OfType<MetadataTableTreeNode>().Count();
 	}

ILSpy.Tests/Metadata/MetadataDisablesLanguageSwitchingTests.cs

@@ -26,11 +26,9 @@ using Avalonia.VisualTree;
 using AwesomeAssertions;
 
 using ILSpy;
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -88,10 +86,7 @@ public class MetadataDisablesLanguageSwitchingTests
 		// DockWorkspace.ActiveContentTabPage.SupportsLanguageSwitching must flip to false.
 		// Selecting a decompilable type after that must flip it back to true.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 
 		var toolbar = await window.WaitForComponent<MainToolBar>();
 		var languageCombo = toolbar.GetVisualDescendants().OfType<ComboBox>()
@@ -103,11 +98,9 @@ public class MetadataDisablesLanguageSwitchingTests
 
 		// Navigate to a metadata node (DOS Header is a stable choice — every PE has one).
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var dosHeader = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
+		var dosHeader = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DosHeaderTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(dosHeader);
 		await Waiters.WaitForAsync(

ILSpy.Tests/Metadata/MetadataFilterTests.cs

@@ -28,10 +28,8 @@ using Avalonia.VisualTree;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.Metadata.CorTables;
-using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
 using ILSpy.Views;
 
@@ -295,19 +293,12 @@ public class MetadataFilterTests
 		// assert on the live DataGridCollectionView's Count (what the user actually sees on
 		// screen) — not on the predicate evaluated over Items, which can pass even when the
 		// CollectionView's Filter is silently null.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var (window, vm) = await TestHarness.BootAsync();
+
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();

ILSpy.Tests/Metadata/MetadataNodeIconParityTests.cs

@@ -24,11 +24,7 @@ using Avalonia.Media;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -47,15 +43,9 @@ public class MetadataNodeIconParityTests
 		// restored per-node parity.
 
 		// Arrange — boot, expand CoreLib's Metadata folder.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 		metadataNode.EnsureLazyChildren();
 		var children = metadataNode.Children;
 

ILSpy.Tests/Metadata/MetadataProtocolHandlerDrillDownTests.cs

@@ -28,7 +28,6 @@ using AwesomeAssertions;
 using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.Metadata.CorTables;
-using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
 using ILSpy.Views;
 
@@ -117,18 +116,11 @@ public class MetadataProtocolHandlerDrillDownTests
 
 	static async Task<MetadataTreeNode> GetMetadataTreeNodeForCoreLib()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 		// Drill-down needs the children realised; EnsureLazyChildren cascades only one level.
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var tablesNode = metadataNode.GetChild<MetadataTablesTreeNode>();
 		tablesNode.EnsureLazyChildren();
 		return metadataNode;
 	}
@@ -137,8 +129,7 @@ public class MetadataProtocolHandlerDrillDownTests
 	{
 		var metadataNode = await GetMetadataTreeNodeForCoreLib();
 		var vm = (MainWindowViewModel)AppComposition.Current.GetExport<MainWindow>().DataContext!;
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = assemblyNode.LoadedAssembly.GetMetadataFileOrNull()!;
 		// The handler is exported as IProtocolHandler, not as itself — fish out the
 		// MetadataProtocolHandler concrete impl from the contract list.

ILSpy.Tests/Metadata/MetadataRowActivationTests.cs

@@ -47,19 +47,12 @@ public class MetadataRowActivationTests
 		// RowActivated, and confirm the assembly tree's TypeTreeNode for System.Object
 		// is now selected — that's the gateway to the decompiler view.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -84,19 +77,12 @@ public class MetadataRowActivationTests
 		// Shift+double-click on a metadata row should NOT replace the active tab — it should
 		// open a brand-new decompiler tab containing the row's entity. The single-tab-reuse
 		// path stays put.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 		var metadataTab = await vm.DockWorkspace.WaitForMetadataTabAsync();

ILSpy.Tests/Metadata/MetadataTabSessionRestoreTests.cs

@@ -25,12 +25,9 @@ using AwesomeAssertions;
 
 using ILSpy;
 using ILSpy.AppEnv;
-using ILSpy.Commands;
 using ILSpy.Metadata;
 using ILSpy.Metadata.CorTables;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -46,19 +43,12 @@ public class MetadataTabSessionRestoreTests
 		// restart (or a tree-path lookup) can land back on the same node. The metadata
 		// sub-tree's ToString() values must be stable for that round-trip to work.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 
@@ -82,40 +72,21 @@ public class MetadataTabSessionRestoreTests
 		// The path-restore must walk through both Metadata folders and land on a debug-only
 		// table.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var testDllPath = typeof(ICSharpCode.Decompiler.Metadata.MetadataFile).Assembly.Location;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var openCommand = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(ICSharpCode.ILSpy.Properties.Resources._Open))
-			.CreateExport().Value;
-		openCommand.Execute(testDllPath);
-
-		await Waiters.WaitForAsync(() =>
-			vm.AssemblyTreeModel.AssemblyList!.GetAssemblies().Any(a =>
-				string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase)));
-		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
-			.First(a => string.Equals(a.FileName, testDllPath, System.StringComparison.OrdinalIgnoreCase));
-		await loaded.GetLoadResultAsync();
+		var loaded = await vm.OpenAssemblyAsync(testDllPath);
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(loaded.ShortName);
-		assemblyNode.EnsureLazyChildren();
 		// AssemblyTreeNode surfaces the embedded PDB's metadata as a second top-level
 		// MetadataTreeNode sibling, so disambiguate by label — this test exercises the
 		// nested-under-DebugDirectory path.
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>()
-			.Single(n => (string?)n.Text == ICSharpCode.ILSpy.Properties.Resources.Metadata);
-		metadataNode.EnsureLazyChildren();
-		var debugDirectoryNode = metadataNode.Children.OfType<DebugDirectoryTreeNode>().Single();
-		debugDirectoryNode.EnsureLazyChildren();
-		var embeddedPdbNode = debugDirectoryNode.Children.OfType<MetadataTreeNode>().Single();
-		embeddedPdbNode.EnsureLazyChildren();
-		var pdbTables = embeddedPdbNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		pdbTables.EnsureLazyChildren();
-		var documentTable = pdbTables.Children.OfType<global::ILSpy.Metadata.DebugTables.DocumentTableTreeNode>().Single();
+		var documentTable = assemblyNode
+			.GetChild<MetadataTreeNode>(n => (string?)n.Text == ICSharpCode.ILSpy.Properties.Resources.Metadata)
+			.GetChild<DebugDirectoryTreeNode>()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<global::ILSpy.Metadata.DebugTables.DocumentTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(documentTable);
 

ILSpy.Tests/Metadata/MetadataTablesTreeTests.cs

@@ -24,11 +24,7 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -45,15 +41,9 @@ public class MetadataTablesTreeTests
 		// Phase 1 lazy-loads its children with a generic placeholder per table; Phase 1e
 		// replaces them with per-table typed leaves carrying the actual row data.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 		metadataNode.EnsureLazyChildren();
 
 		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
@@ -68,17 +58,11 @@ public class MetadataTablesTreeTests
 	[AvaloniaTest]
 	public async Task MetadataTablesTreeNode_LazyLoads_One_Child_Per_NonEmpty_Table()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var tablesNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
 		tablesNode.EnsureLazyChildren();
 
 		var tableChildren = tablesNode.Children.OfType<MetadataTableTreeNode>().ToList();
@@ -93,17 +77,11 @@ public class MetadataTablesTreeTests
 	[AvaloniaTest]
 	public async Task MetadataTablesTreeNode_Decompiles_To_A_Per_Table_Row_Count_Summary()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var tablesNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(tablesNode);
 		var tab = await vm.DockWorkspace.WaitForDecompiledTextAsync();

ILSpy.Tests/Metadata/MetadataTokenNavigationTests.cs

@@ -24,10 +24,8 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.Metadata.CorTables;
-using ILSpy.TreeNodes;
 using ILSpy.ViewModels;
 using ILSpy.Views;
 
@@ -45,19 +43,12 @@ public class MetadataTokenNavigationTests
 		// must dispatch through the docking host to the TypeRef / TypeDef / TypeSpec table
 		// the token belongs to and scroll the receiving grid to the referenced row.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -94,19 +85,12 @@ public class MetadataTokenNavigationTests
 		// entry. It should fire the same NavigateToCellRequested path as the hyperlink-style
 		// click, with the focused (or last-clicked) token cell as the target.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var (window, vm) = await TestHarness.BootAsync();
+
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(typeDefNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();

ILSpy.Tests/Metadata/MetadataTreeShapeTests.cs

@@ -23,11 +23,8 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -45,17 +42,12 @@ public class MetadataTreeShapeTests
 		// MetadataTreeNode instance ready to lazy-load its children.
 
 		// Arrange — boot, wait for assemblies, expand CoreLib (always a PE).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 
 		// Assert — exactly one MetadataTreeNode child and it precedes the References folder.
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var metadataNode = assemblyNode.GetChild<MetadataTreeNode>();
 		var children = assemblyNode.Children.ToList();
 		children.IndexOf(metadataNode).Should().BeLessThan(
 			children.IndexOf(assemblyNode.Children.OfType<ReferenceFolderTreeNode>().Single()),
@@ -71,15 +63,9 @@ public class MetadataTreeShapeTests
 		// pipeline rather than introducing a new tab type for v1.
 
 		// Arrange — boot, find the Metadata folder under CoreLib.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 
 		// Act — select it through the helper that mirrors a click (loads lazy children + assigns
 		// SelectedItem), wait for the decompile to finish.

ILSpy.Tests/Metadata/PEHeaderTreeTests.cs

@@ -23,11 +23,7 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -44,15 +40,9 @@ public class PEHeaderTreeTests
 		// user shifting between hosts sees the same tree shape. Each node is the entry point
 		// to a header / table view; Phase 1 renders text, Phase 2 swaps to a DataGrid.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 		metadataNode.EnsureLazyChildren();
 
 		metadataNode.Children.OfType<DosHeaderTreeNode>().Should().ContainSingle();
@@ -78,17 +68,11 @@ public class PEHeaderTreeTests
 		// row per field, columns reflected from the Entry shape (Member / Offset / Size /
 		// Value / Meaning).
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
+		var dosNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DosHeaderTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(dosNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -113,17 +97,12 @@ public class PEHeaderTreeTests
 		// decompiler tab but never restored it as the user-visible page. Both round-trip
 		// directions should swap the active dockable to the right tab type.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
+		var dosNode = assemblyNode
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DosHeaderTreeNode>();
 
 		// Step 1 — pick a regular entity node (the assembly itself decompiles to its
 		// AssemblyDef metadata via the decompiler text path). Decompiler tab should be active.
@@ -151,17 +130,12 @@ public class PEHeaderTreeTests
 		// the docking host can lazily create and activate the decompiler tab even when the
 		// document area starts out holding only the metadata grid.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
+		var dosNode = assemblyNode
+			.GetChild<MetadataTreeNode>()
+			.GetChild<DosHeaderTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(dosNode);
 		await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -183,18 +157,12 @@ public class PEHeaderTreeTests
 		// previous inner view rendered (the user would see a "Decompiling" spinner that
 		// never goes away because the prior decompiler tab is still on screen).
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
-		var coffNode = metadataNode.Children.OfType<CoffHeaderTreeNode>().Single();
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
+		var metadataNode = assemblyNode.GetChild<MetadataTreeNode>();
+		var dosNode = metadataNode.GetChild<DosHeaderTreeNode>();
+		var coffNode = metadataNode.GetChild<CoffHeaderTreeNode>();
 		var factoryFactory = (global::ILSpy.Docking.ILSpyDockFactory)vm.DockWorkspace.Factory;
 		var documents = factoryFactory.Documents!;
 		var mainTab = factoryFactory.MainTab!;
@@ -228,19 +196,12 @@ public class PEHeaderTreeTests
 		// decompiler tab reuses itself across tree-node selections. Otherwise every click
 		// piles up a new dockable and the tab strip grows without bound.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
+		var metadataNode = vm.AssemblyTreeModel.FindCoreLib().GetChild<MetadataTreeNode>();
 
-		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
-		var coffNode = metadataNode.Children.OfType<CoffHeaderTreeNode>().Single();
+		var dosNode = metadataNode.GetChild<DosHeaderTreeNode>();
+		var coffNode = metadataNode.GetChild<CoffHeaderTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(dosNode);
 		var firstTab = await vm.DockWorkspace.WaitForMetadataTabAsync();
@@ -263,17 +224,11 @@ public class PEHeaderTreeTests
 	[AvaloniaTest]
 	public async Task CoffHeaderTreeNode_Opens_A_DataGrid_Tab_With_Machine_And_Characteristics_Rows()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var coffNode = metadataNode.Children.OfType<CoffHeaderTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var coffNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<CoffHeaderTreeNode>();
 
 		vm.AssemblyTreeModel.SelectNode(coffNode);
 		var tab = await vm.DockWorkspace.WaitForMetadataTabAsync();

ILSpy.Tests/Metadata/ShowInMetadataContextMenuTests.cs

@@ -27,14 +27,9 @@ using AwesomeAssertions;
 using ICSharpCode.Decompiler.TypeSystem;
 
 using ILSpy;
-using ILSpy.AppEnv;
 using ILSpy.Commands;
 using ILSpy.Metadata;
-using ILSpy.Metadata.CorTables;
 using ILSpy.TextView;
-using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -51,13 +46,9 @@ public class ShowInMetadataContextMenuTests
 		// verify the entry's IsVisible/Execute drives the dock workspace into the right
 		// metadata table for the entity's MetadataToken.kind.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 
 		// Resolve System.Object's IEntity via the type system, since the entry only acts
 		// on IEntity-bearing references.

ILSpy.Tests/Metadata/TypedMetadataTableTreeTests.cs

@@ -24,12 +24,9 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Metadata;
 using ILSpy.Metadata.CorTables;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -46,18 +43,12 @@ public class TypedMetadataTableTreeTests
 		// tab whose Items list mirrors the table's rows; a known reference's display name
 		// must show up on at least one row.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var assemblyRefNode = tablesNode.Children.OfType<AssemblyRefTableTreeNode>().Single();
+		var assemblyRefNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq")
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<AssemblyRefTableTreeNode>();
 
 		assemblyRefNode.Kind.Should().Be(TableIndex.AssemblyRef);
 		assemblyRefNode.RowCount.Should().BeGreaterThan(0);
@@ -76,19 +67,12 @@ public class TypedMetadataTableTreeTests
 	[AvaloniaTest]
 	public async Task TypeDefTableTreeNode_Opens_A_Grid_Including_System_Object()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var typeDefNode = tablesNode.Children.OfType<TypeDefTableTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var typeDefNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<TypeDefTableTreeNode>();
 
 		typeDefNode.Kind.Should().Be(TableIndex.TypeDef);
 		typeDefNode.RowCount.Should().BeGreaterThan(1000);
@@ -107,17 +91,11 @@ public class TypedMetadataTableTreeTests
 		// Field / MethodDef are by far the largest tables in any real assembly. The typed
 		// leaves should report the right kind and expose row data through the grid view.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var tablesNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>();
 		tablesNode.EnsureLazyChildren();
 
 		tablesNode.Children.OfType<FieldTableTreeNode>().Should().ContainSingle();
@@ -137,19 +115,12 @@ public class TypedMetadataTableTreeTests
 	[AvaloniaTest]
 	public async Task ModuleTableTreeNode_Opens_A_Grid_With_The_Single_Module_Row()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
-		assemblyNode.EnsureLazyChildren();
-		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
-		metadataNode.EnsureLazyChildren();
-		var tablesNode = metadataNode.Children.OfType<MetadataTablesTreeNode>().Single();
-		tablesNode.EnsureLazyChildren();
-		var moduleNode = tablesNode.Children.OfType<ModuleTableTreeNode>().Single();
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var moduleNode = vm.AssemblyTreeModel.FindCoreLib()
+			.GetChild<MetadataTreeNode>()
+			.GetChild<MetadataTablesTreeNode>()
+			.GetChild<ModuleTableTreeNode>();
 
 		moduleNode.RowCount.Should().Be(1, "Module is a one-row table by spec");
 

ILSpy.Tests/Navigation/BrowseBackForwardCommandTests.cs

@@ -79,10 +79,7 @@ public class BrowseBackForwardCommandTests
 		// Execute pops one entry. Verifies via the live MenuItem that ends up in the View menu.
 
 		// Arrange — boot the window, load assemblies, expand Enumerable, capture two methods.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");

ILSpy.Tests/Navigation/NavigationTests.cs

@@ -25,10 +25,7 @@ using Avalonia.VisualTree;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -46,10 +43,7 @@ public class NavigationTests
 
 		// Arrange — boot the window, wait for assemblies, expand Enumerable, capture two methods
 		// that we'll bounce between.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -98,10 +92,7 @@ public class NavigationTests
 		// displaced entries pushed onto the forward stack).
 
 		// Arrange — boot, wait for assemblies, expand Enumerable, capture three methods.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
@@ -167,10 +158,7 @@ public class NavigationTests
 
 		// Arrange — boot, wait for assemblies, expand System.Exception so we can select its
 		// BaseTypesTreeNode child.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(

ILSpy.Tests/Navigation/ViewStateRoundTripTests.cs

@@ -23,12 +23,9 @@ using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
-using ILSpy.AppEnv;
 using ILSpy.Navigation;
 using ILSpy.TextView;
 using ILSpy.TreeNodes;
-using ILSpy.ViewModels;
-using ILSpy.Views;
 
 using NUnit.Framework;
 
@@ -47,10 +44,7 @@ public class ViewStateRoundTripTests
 	[AvaloniaTest]
 	public async Task Back_Restores_Caret_Position_The_User_Left_On_The_Previous_Node()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var dockWorkspace = vm.DockWorkspace;
 
@@ -111,10 +105,7 @@ public class ViewStateRoundTripTests
 		// a deterministic snapshot, exercise the navigation pipeline, and assert the snapshot
 		// survives the capture -> entry -> pending hops.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var dockWorkspace = vm.DockWorkspace;
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
@@ -165,10 +156,7 @@ public class ViewStateRoundTripTests
 		// as missing, but it was already implemented. This test makes a regression in any
 		// of the three wirings (command, toolbar button, Alt+Right) detectable.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		((object?)vm.DockWorkspace.NavigateForwardCommand).Should().NotBeNull();
 		((object?)vm.DockWorkspace.NavigateBackCommand).Should().NotBeNull();

ILSpy.Tests/Options/ApiVisibilityFilterTests.cs

@@ -153,10 +153,7 @@ public class ApiVisibilityFilterTests
 		// re-evaluates visibility when ShowApiLevel changes.
 
 		// Arrange — boot, find a CoreLib type with mixed accessibility (String has many).
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		var coreLibName = typeof(object).Assembly.GetName().Name!;
 		var stringType = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(coreLibName, "System", "System.String");
@@ -187,10 +184,7 @@ public class ApiVisibilityFilterTests
 		// new HierarchicalModel reference whenever the setting changes.
 
 		// Arrange — boot, locate the pane.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, _) = await TestHarness.BootAsync();
 		await Waiters.WaitForAsync(
 			() => window.GetVisualDescendants().OfType<AssemblyListPane>().Any());
 		var pane = await window.WaitForComponent<AssemblyListPane>();
@@ -219,10 +213,7 @@ public class ApiVisibilityFilterTests
 		// than Foreground brush) avoids depending on theme brush resolution.
 
 		// Arrange — boot, ensure ShowApiLevel=All so non-public methods are visible at all.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, vm) = await TestHarness.BootAsync();
 		var settings = AppComposition.Current.GetExport<SettingsService>().SessionSettings.LanguageSettings;
 		settings.ShowApiLevel = ApiVisibility.All;
 
@@ -308,10 +299,7 @@ public class ApiVisibilityFilterTests
 
 	static async Task<(MainWindowViewModel vm, TypeTreeNode enumerableNode)> BootAndExpandEnumerableAsync()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 		var node = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
 			"System.Linq", "System.Linq", "System.Linq.Enumerable");
 		node.IsExpanded = true;

ILSpy.Tests/Options/OptionsPageScrollReachTests.cs

@@ -58,10 +58,8 @@ public class OptionsPageScrollReachTests
 		window.Show();
 		Dispatcher.UIThread.RunJobs();
 
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var command = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value;
+		var command = AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options));
 		command.Execute(null);
 		Dispatcher.UIThread.RunJobs();
 

ILSpy.Tests/Options/OptionsTabTests.cs

@@ -66,10 +66,8 @@ public class OptionsTabTests
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
 		var vm = (MainWindowViewModel)window.DataContext!;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var command = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value;
+		var command = AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options));
 
 		command.Execute(null);
 
@@ -92,15 +90,11 @@ public class OptionsTabTests
 		// document is still Options — so the user sees no visible change. The fix:
 		// ShowSelectedNode pulls focus back to MainTab whenever a tree-selection writes
 		// new content there.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+		var (_, vm) = await TestHarness.BootAsync(3);
 
 		// Open Options and confirm it's the active document.
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		registry.Commands.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value.Execute(null);
+		AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options)).Execute(null);
 
 		var documents = vm.DockWorkspace.Documents!;
 		var optionsTab = documents.VisibleDockables!.OfType<ContentTabPage>()
@@ -133,9 +127,8 @@ public class OptionsTabTests
 		// tab should use the bare Resources.Options ("Options") instead.
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		registry.Commands.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value.Execute(null);
+		AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options)).Execute(null);
 
 		var vm = (MainWindowViewModel)window.DataContext!;
 		var model = (OptionsPageModel)vm.DockWorkspace.Documents!.VisibleDockables!
@@ -157,10 +150,8 @@ public class OptionsTabTests
 		// byte-for-byte.
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var command = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value;
+		var command = AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options));
 		command.Execute(null);
 
 		var vm = (MainWindowViewModel)window.DataContext!;
@@ -181,10 +172,8 @@ public class OptionsTabTests
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
 		var vm = (MainWindowViewModel)window.DataContext!;
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		var command = registry.Commands
-			.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value;
+		var command = AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options));
 
 		command.Execute(null);
 		command.Execute(null);
@@ -205,12 +194,11 @@ public class OptionsTabTests
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
 		var settings = AppComposition.Current.GetExport<SettingsService>();
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
 
 		var liveBefore = settings.DecompilerSettings.UsingDeclarations;
 
-		registry.Commands.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value.Execute(null);
+		AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options)).Execute(null);
 
 		var vm = (MainWindowViewModel)window.DataContext!;
 		var model = (OptionsPageModel)vm.DockWorkspace.Documents!.VisibleDockables!
@@ -238,9 +226,8 @@ public class OptionsTabTests
 		// default; Display and Misc panels are untouched (verified by leaving them alone).
 		var window = AppComposition.Current.GetExport<MainWindow>();
 		window.Show();
-		var registry = AppComposition.Current.GetExport<MainMenuCommandRegistry>();
-		registry.Commands.Single(c => c.Metadata.Header == nameof(Resources._Options))
-			.CreateExport().Value.Execute(null);
+		AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+			.GetCommand(nameof(Resources._Options)).Execute(null);
 
 		var vm = (MainWindowViewModel)window.DataContext!;
 		var model = (OptionsPageModel)vm.DockWorkspace.Documents!.VisibleDockables!

ILSpy.Tests/Search/ScopeSearchToTests.cs

@@ -43,15 +43,10 @@ public class ScopeSearchToTests
 	[AvaloniaTest]
 	public async Task Scope_Search_To_Assembly_Prepends_InAssembly_To_The_Search_Term()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources.ScopeSearchToThisAssembly))
-			.Value;
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry(nameof(Resources.ScopeSearchToThisAssembly));
 
 		var asm = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
@@ -71,15 +66,10 @@ public class ScopeSearchToTests
 	[AvaloniaTest]
 	public async Task Scope_Search_To_Namespace_Prepends_InNamespace_To_The_Search_Term()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
-
-		var registry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>();
-		var entry = registry.Entries
-			.Single(e => e.Metadata.Header == nameof(Resources.ScopeSearchToThisNamespace))
-			.Value;
+		var (_, vm) = await TestHarness.BootAsync();
+
+		var entry = AppComposition.Current.GetExport<ContextMenuEntryRegistry>()
+			.GetEntry(nameof(Resources.ScopeSearchToThisNamespace));
 
 		var ns = vm.AssemblyTreeModel.FindNode<NamespaceTreeNode>("System.Linq", "System.Linq");
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();

ILSpy.Tests/Search/SearchInputFocusTests.cs

@@ -44,10 +44,7 @@ public class SearchInputFocusTests
 		// can start typing immediately. The VM raises FocusRequested; the SearchPane
 		// code-behind subscribes and calls Focus() on the TextBox.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();
@@ -64,10 +61,7 @@ public class SearchInputFocusTests
 	[AvaloniaTest]
 	public async Task SearchInput_Is_Focused_After_ShowSearchCommand_Pumps_Through_The_Dispatcher()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (window, _) = await TestHarness.BootAsync();
 
 		var pane = await window.WaitForComponent<SearchPane>();
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();

ILSpy.Tests/Search/SearchPaneAssemblyListChangedTests.cs

@@ -48,10 +48,7 @@ public class SearchPaneAssemblyListChangedTests
 	[AvaloniaTest]
 	public async Task Search_Restarts_When_A_User_Loaded_Assembly_Is_Added()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.SearchTerm = "Object";
@@ -82,10 +79,7 @@ public class SearchPaneAssemblyListChangedTests
 		// Auto-loaded dependencies fire from result navigation in a large assembly; restarting
 		// the search there would feed back into more loads → more events → flicker. The
 		// handler MUST take the early-out path.
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.SearchTerm = "Object";

ILSpy.Tests/Search/SearchPaneStreamingTests.cs

@@ -45,13 +45,9 @@ public class SearchPaneStreamingTests
 		// logic works for the fixture. If this fails, the bug is in the strategy / fixture;
 		// if it passes but the orchestrator test below fails, the bug is in our wiring.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
-		var coreLibName = typeof(object).Assembly.GetName().Name!;
-		var assemblyNode = vm.AssemblyTreeModel.FindNode<global::ILSpy.TreeNodes.AssemblyTreeNode>(coreLibName);
+		var assemblyNode = vm.AssemblyTreeModel.FindCoreLib();
 		var module = await assemblyNode.LoadedAssembly.GetMetadataFileAsync();
 
 		var queue = new System.Collections.Concurrent.ConcurrentQueue<ICSharpCode.ILSpyX.Search.SearchResult>();
@@ -82,10 +78,7 @@ public class SearchPaneStreamingTests
 		// search across the loaded fixture assemblies, and wait for at least one matching
 		// result to land in the Results collection.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.Results.Clear();
@@ -107,10 +100,7 @@ public class SearchPaneStreamingTests
 		// Switching to an empty term tears down the running search and clears the visible
 		// results. The user shouldn't see stale rows after they've explicitly cleared the box.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.Results.Clear();
@@ -131,10 +121,7 @@ public class SearchPaneStreamingTests
 		// contain". Confirm end-to-end: a Type search for "String -Builder" returns names
 		// containing "String" (System.String, StringComparer, …) but never StringBuilder.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.Results.Clear();
@@ -159,10 +146,7 @@ public class SearchPaneStreamingTests
 		// (length-aware; ignores IL backtick generic arity). Confirm: =String must match
 		// System.String but NOT StringBuilder or StringComparer.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.Results.Clear();

ILSpy.Tests/Search/SearchProgressTests.cs

@@ -44,10 +44,7 @@ public class SearchProgressTests
 	[AvaloniaTest]
 	public async Task IsSearching_Flips_True_When_A_Search_Starts_And_False_When_It_Finishes()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.SearchTerm = string.Empty;
@@ -84,10 +81,7 @@ public class SearchProgressTests
 	[AvaloniaTest]
 	public async Task SearchResult_Image_Properties_Are_Non_Null_So_Row_Icons_Render()
 	{
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.SearchTerm = string.Empty;

ILSpy.Tests/Search/SearchResultNavigationTests.cs

@@ -46,10 +46,7 @@ public class SearchResultNavigationTests
 		// to activate it, and confirm the assembly tree's SelectedItem becomes the matching
 		// tree node.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (_, vm) = await TestHarness.BootAsync();
 
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();
 		search.Results.Clear();

ILSpy.Tests/Search/SearchResultSortOrderTests.cs

@@ -47,10 +47,7 @@ public class SearchResultSortOrderTests
 		// Name asc (StringComparer.Ordinal). Mirrors WPF's SearchPane.xaml.cs:288-290
 		// which captures the comparer at start-of-search based on DisplaySettings.SortResults.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var settings = AppComposition.Current.GetExport<SettingsService>();
 		var originalSortResults = settings.DisplaySettings.SortResults;

ILSpy.Tests/Search/ShowSearchCommandTests.cs

@@ -42,10 +42,7 @@ public class ShowSearchCommandTests
 		// MainWindow.KeyBindings. Here we drive the command directly and verify the
 		// activation reaches the SearchPaneModel.
 
-		var window = AppComposition.Current.GetExport<MainWindow>();
-		window.Show();
-		var vm = (MainWindowViewModel)window.DataContext!;
-		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		await TestHarness.BootAsync();
 
 		var dockWorkspace = AppComposition.Current.GetExport<DockWorkspace>();
 		var search = AppComposition.Current.GetExport<SearchPaneModel>();

ILSpy.Tests/TestHarness.cs

@@ -0,0 +1,112 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows.Input;
+
+using ICSharpCode.ILSpy.Properties;
+using ICSharpCode.ILSpyX;
+
+using global::ILSpy;
+using global::ILSpy.AppEnv;
+using global::ILSpy.Commands;
+using global::ILSpy.ViewModels;
+using global::ILSpy.Views;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// Boots and drives the shared MEF-composed application for headless UI tests, collapsing the
+/// four-line prologue (resolve <see cref="MainWindow"/>, show it, cast its DataContext, wait for
+/// the assembly list) and the "open an assembly via the production command and wait for it to
+/// load" dance that nearly every fixture repeats. The container is rebuilt per test by
+/// <see cref="ResetAppStateAttribute"/>, so each call hands back that test's own singletons.
+/// </summary>
+public static class TestHarness
+{
+	/// <summary>
+	/// Resolves the shared <see cref="MainWindow"/>, shows it, and waits until the default
+	/// assembly list has loaded at least <paramref name="minimumAssemblies"/> assemblies.
+	/// Returns the window and its <see cref="MainWindowViewModel"/> — the two handles the test
+	/// body almost always needs next.
+	/// </summary>
+	public static async Task<(MainWindow Window, MainWindowViewModel ViewModel)> BootAsync(
+		int minimumAssemblies = 1,
+		TimeSpan? timeout = null)
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumAssemblies, timeout);
+		return (window, vm);
+	}
+
+	/// <summary>
+	/// Looks up a [ExportMainMenuCommand]-tagged command by its <c>Header</c> metadata (pass the
+	/// resource key, e.g. <c>nameof(Resources._Open)</c>) and materialises a fresh instance.
+	/// </summary>
+	public static ICommand GetCommand(this MainMenuCommandRegistry registry, string header)
+	{
+		ArgumentNullException.ThrowIfNull(registry);
+		return registry.Commands
+			.Single(c => string.Equals(c.Metadata.Header, header, StringComparison.Ordinal))
+			.CreateExport().Value;
+	}
+
+	/// <summary>
+	/// Looks up a [ExportContextMenuEntry]-tagged entry by its <c>Header</c> metadata.
+	/// </summary>
+	public static IContextMenuEntry GetEntry(this ContextMenuEntryRegistry registry, string header)
+	{
+		ArgumentNullException.ThrowIfNull(registry);
+		return registry.Entries
+			.Single(e => string.Equals(e.Metadata.Header, header, StringComparison.Ordinal))
+			.Value;
+	}
+
+	/// <summary>
+	/// Opens <paramref name="path"/> through the production Open command, waits for it to appear
+	/// in the assembly list, awaits its load result, and returns the resulting
+	/// <see cref="LoadedAssembly"/>. This is the canonical "add a test assembly to the tree"
+	/// sequence — registry lookup, execute, poll-for-appearance, await-load — in one call.
+	/// </summary>
+	public static async Task<LoadedAssembly> OpenAssemblyAsync(
+		this MainWindowViewModel vm,
+		string path,
+		TimeSpan? timeout = null)
+	{
+		ArgumentNullException.ThrowIfNull(vm);
+		ArgumentNullException.ThrowIfNull(path);
+
+		var open = AppComposition.Current.GetExport<MainMenuCommandRegistry>().GetCommand(nameof(Resources._Open));
+		open.Execute(path);
+
+		await Waiters.WaitForAsync(
+			() => vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
+				.Any(a => string.Equals(a.FileName, path, StringComparison.OrdinalIgnoreCase)),
+			timeout,
+			$"assembly '{path}' to appear in the active list");
+
+		var loaded = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
+			.First(a => string.Equals(a.FileName, path, StringComparison.OrdinalIgnoreCase));
+		await loaded.GetLoadResultAsync();
+		return loaded;
+	}
+}

ILSpy.Tests/TreeNavigation.cs

@@ -22,6 +22,7 @@ using System.Linq;
 using ICSharpCode.ILSpyX.TreeView;
 
 using global::ILSpy.AssemblyTree;
+using global::ILSpy.TreeNodes;
 
 namespace ICSharpCode.ILSpy.Tests;
 
@@ -36,6 +37,56 @@ namespace ICSharpCode.ILSpy.Tests;
 /// </summary>
 public static class TreeNavigation
 {
+	/// <summary>
+	/// Short name of the runtime core library (e.g. <c>System.Private.CoreLib</c>) — the one
+	/// assembly the headless suite is guaranteed to have loaded. Use as the first path segment.
+	/// </summary>
+	public static string CoreLibName { get; } = typeof(object).Assembly.GetName().Name!;
+
+	/// <summary>
+	/// Resolves the core-library <see cref="AssemblyTreeNode"/>, the fixture every test can rely
+	/// on. Replaces the repeated <c>FindNode&lt;AssemblyTreeNode&gt;(typeof(object).Assembly...)</c>.
+	/// </summary>
+	internal static AssemblyTreeNode FindCoreLib(this AssemblyTreeModel atm)
+		=> atm.FindNode<AssemblyTreeNode>(CoreLibName);
+
+	/// <summary>
+	/// Materialises <paramref name="parent"/>'s lazy children and returns the single child of type
+	/// <typeparamref name="T"/>. The workhorse for drilling metadata sub-trees, replacing the
+	/// <c>node.EnsureLazyChildren(); node.Children.OfType&lt;T&gt;().Single()</c> pair. Chains.
+	/// </summary>
+	public static T GetChild<T>(this SharpTreeNode parent) where T : SharpTreeNode
+	{
+		ArgumentNullException.ThrowIfNull(parent);
+		parent.EnsureLazyChildren();
+		return parent.Children.OfType<T>().Single();
+	}
+
+	/// <summary>
+	/// Materialises lazy children and returns the single child of type <typeparamref name="T"/>
+	/// matching <paramref name="predicate"/> (e.g. a method picked by name).
+	/// </summary>
+	public static T GetChild<T>(this SharpTreeNode parent, Func<T, bool> predicate) where T : SharpTreeNode
+	{
+		ArgumentNullException.ThrowIfNull(parent);
+		ArgumentNullException.ThrowIfNull(predicate);
+		parent.EnsureLazyChildren();
+		return parent.Children.OfType<T>().Single(predicate);
+	}
+
+	/// <summary>
+	/// Materialises lazy children, flips <see cref="SharpTreeNode.IsExpanded"/>, and returns the
+	/// node so callers can keep drilling. Replaces the
+	/// <c>node.EnsureLazyChildren(); node.IsExpanded = true;</c> pair.
+	/// </summary>
+	public static T Expand<T>(this T node) where T : SharpTreeNode
+	{
+		ArgumentNullException.ThrowIfNull(node);
+		node.EnsureLazyChildren();
+		node.IsExpanded = true;
+		return node;
+	}
+
 	public static SharpTreeNode FindNode(this AssemblyTreeModel atm, params string[] segments)
 	{
 		ArgumentNullException.ThrowIfNull(atm);