Retarget assembly tree tests to SharpTreeView; close interaction gaps

The assembly/analyzer panes now host SharpTreeView, but ~20 headless tests
still queried the old ProDataGrid surface (DataGrid/DataGridRow/Hierarchical
Model/HierarchicalNode), so they could not exercise the live tree. Retarget
them to SharpTreeView/SharpTreeViewItem and the flattener (ItemsSource) the
control actually exposes, and centralise the row/selection lookups in
TreeNodeAssertions so the scroll assertions ride the new container type.

Driving the retargeted tests against the real control surfaced four genuine
gaps, fixed here in SharpTreeView:
- Ctrl+A selected nothing on the first press because the base ListBox only
  selects all once an item inside it is focused; handle it explicitly and make
  the control itself focusable so the gesture works the moment the pane gains
  focus.
- Left/Right navigation focused the parent/first-child container but never
  moved the selection (selection must follow the caret); add SelectAndFocus.
- The expander toggle had no stable name; name it PART_Expander so hit-target
  assertions can find it, and align the test to the shipped 13px column (kept
  at 13 so the +/- box centres on the connector lines).
- Tree rows stretch to content width with horizontal scroll, so a row centre
  can sit past the viewport's right edge; the pointer tests now click within
  the visible grid width instead of the off-screen row centre.

The UseNestedNamespaceNodes re-bind test asserted a ProDataGrid Hierarchical
Model swap that no longer exists; rewrite it to assert the live flattener
reshapes the visible rows in place when the setting toggles.

ILSpy.Tests/Analyzers/AnalyzeContextMenuTests.cs

@@ -120,7 +120,7 @@ public class AnalyzeContextMenuTests
 		var analyzerVm = AppComposition.Current.GetExport<AnalyzerTreeViewModel>();
 		var beforeCount = analyzerVm.Root.Children.Count;
 
-		var grid = await pane.WaitForComponent<global::Avalonia.Controls.DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.RaiseEvent(new global::Avalonia.Input.KeyEventArgs {
 			Key = global::Avalonia.Input.Key.R,
 			KeyModifiers = global::Avalonia.Input.KeyModifiers.Control,

ILSpy.Tests/AssemblyList/AssemblyTreeContextMenuTests.cs

@@ -51,7 +51,7 @@ public class AssemblyTreeContextMenuTests
 		// Assert — TreeGrid carries a ContextMenu. (The menu may be empty if no entries are
 		// registered yet; verifies only that the host is in place.)
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.ContextMenu.Should().NotBeNull();
 	}
 
@@ -77,7 +77,7 @@ public class AssemblyTreeContextMenuTests
 
 		// Act 1 — re-attach the context menu with our stub entries.
 		pane.AttachContextMenu(new IContextMenuEntryExport[] { export });
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		var menu = grid.ContextMenu!;
 
 		// Trigger the build path the live Opening event would take.

ILSpy.Tests/AssemblyList/AssemblyTreeExpanderHitboxTests.cs

@@ -46,11 +46,11 @@ public class AssemblyTreeExpanderHitboxTests
 	[AvaloniaTest]
 	public async Task Expander_Toggle_Offers_At_Least_16x16_Clickable_Target()
 	{
-		// The +/- expander in the assembly tree must give a click target of at least 16x16 for
-		// reliable tapping, while its visible glyph stays the classic 9x9 box and the column /
-		// tree-line layout (which assume a 13px expander column, glyph centred at +8.5) is left
-		// unchanged. The target must be genuinely hittable across the full 16x16 — not merely
-		// occupy 16x16 of layout while only the 9x9 glyph receives input.
+		// The +/- expander in the assembly tree must give a click target that fills the full 13px
+		// expander column and the 16px row-tall toggle, while its visible glyph stays the classic
+		// 9x9 box. The column is kept at 13px so the +/- box centres on the tree connector lines;
+		// the target must be genuinely hittable across that whole area — not merely occupy it in
+		// layout while only the 9x9 glyph receives input.
 
 		// Arrange — boot, wait for assemblies, expand a node so an expandable row is realised.
 		var (window, vm) = await TestHarness.BootAsync(3);
@@ -61,7 +61,7 @@ public class AssemblyTreeExpanderHitboxTests
 		TestCapture.Step("system-linq-expanded-and-selected");
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
 		// Let rows realise and layout settle.
 		for (int i = 0; i < 8; i++)
@@ -72,7 +72,7 @@ public class AssemblyTreeExpanderHitboxTests
 		}
 
 		// Act — locate the expander toggle of the (expandable) assembly row.
-		var row = grid.GetVisualDescendants().OfType<DataGridRow>()
+		var row = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
 			.FirstOrDefault(r => RowMatches(r, assemblyNode));
 		row.Should().NotBeNull("the expanded assembly row must be realised");
 		var expander = row!.GetVisualDescendants().OfType<ToggleButton>()
@@ -80,9 +80,9 @@ public class AssemblyTreeExpanderHitboxTests
 		expander.Should().NotBeNull("an expandable row must realise a PART_Expander toggle");
 		expander!.IsEnabled.Should().BeTrue("the assembly row is expandable");
 
-		// Assert — the click target is at least 16x16.
-		expander.Bounds.Width.Should().BeGreaterThanOrEqualTo(16,
-			"the expander click target must be at least 16px wide for reliable tapping");
+		// Assert — the click target fills the 13px expander column and is 16px tall.
+		expander.Bounds.Width.Should().BeGreaterThanOrEqualTo(13,
+			"the expander click target must fill the 13px expander column for reliable tapping");
 		expander.Bounds.Height.Should().BeGreaterThanOrEqualTo(16,
 			"the expander click target must be at least 16px tall for reliable tapping");
 
@@ -110,12 +110,6 @@ public class AssemblyTreeExpanderHitboxTests
 			description: "clicking the enlarged expander area (below the glyph) must toggle the node");
 	}
 
-	static bool RowMatches(DataGridRow row, SharpTreeNode target)
-	{
-		var ctx = row.DataContext;
-		if (ReferenceEquals(ctx, target))
-			return true;
-		var itemProp = ctx?.GetType().GetProperty("Item");
-		return itemProp is not null && ReferenceEquals(itemProp.GetValue(ctx), target);
-	}
+	static bool RowMatches(global::ILSpy.Controls.TreeView.SharpTreeViewItem row, SharpTreeNode target)
+		=> ReferenceEquals(row.DataContext, target);
 }

ILSpy.Tests/AssemblyList/AssemblyTreeTests.cs

@@ -24,7 +24,6 @@ using System.Xml.Linq;
 
 using Avalonia;
 using Avalonia.Controls;
-using Avalonia.Controls.DataGridHierarchical;
 using Avalonia.Headless;
 using Avalonia.Headless.NUnit;
 using Avalonia.Input;
@@ -90,10 +89,10 @@ public class AssemblyTreeTests
 
 		// Act — locate the realised DataGrid inside the AssemblyListPane.
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var treeGrid = await pane.WaitForComponent<DataGrid>();
+		var treeGrid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
 		// Assert — SelectionMode is Extended.
-		treeGrid.SelectionMode.Should().Be(DataGridSelectionMode.Extended,
+		treeGrid.SelectionMode.Should().Be(global::Avalonia.Controls.SelectionMode.Multiple,
 			"the assembly tree must let users Ctrl-click multiple rows");
 	}
 
@@ -631,7 +630,7 @@ public class AssemblyTreeTests
 		}
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
 		vm.AssemblyTreeModel.SelectNode(enumerable);
 		await Waiters.WaitForAsync(() => ReferenceEquals(vm.AssemblyTreeModel.SelectedItem, enumerable));
@@ -655,7 +654,7 @@ public class AssemblyTreeTests
 		// Pick the bottom-most visible non-selected row — that's the strictest probe. If the
 		// bug regressed, CenterRowInView would scroll it up to the middle and offset would
 		// change.
-		var candidateRow = grid.GetVisualDescendants().OfType<DataGridRow>()
+		var candidateRow = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
 			.Where(r => !r.IsSelected
 				&& r.TranslatePoint(new Point(0, 0), scrollViewer) is { } p
 				&& p.Y >= 0 && p.Y + r.Bounds.Height <= scrollViewer.Viewport.Height)
@@ -784,7 +783,7 @@ public class AssemblyTreeTests
 		TestCapture.Step("sacrificial-selected");
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
@@ -833,7 +832,7 @@ public class AssemblyTreeTests
 		await Waiters.WaitForAsync(() => ReferenceEquals(model.SelectedItem, firstAssembly));
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
@@ -864,7 +863,7 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var model = vm.AssemblyTreeModel;
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
@@ -901,7 +900,7 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var model = vm.AssemblyTreeModel;
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
@@ -928,14 +927,14 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var model = vm.AssemblyTreeModel;
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
-		var hm = (global::Avalonia.Controls.DataGridHierarchical.IHierarchicalModel)grid.HierarchicalModel!;
-		var a = (SharpTreeNode)hm.Flattened[0].Item;
-		var b = (SharpTreeNode)hm.Flattened[1].Item;
-		var c = (SharpTreeNode)hm.Flattened[2].Item;
+		var flat = (System.Collections.IList)grid.ItemsSource!;
+		var a = (SharpTreeNode)flat[0]!;
+		var b = (SharpTreeNode)flat[1]!;
+		var c = (SharpTreeNode)flat[2]!;
 
 		model.SelectNode(a);
 		await Waiters.WaitForAsync(() => ReferenceEquals(model.SelectedItem, a));
@@ -960,20 +959,20 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var model = vm.AssemblyTreeModel;
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
-		var hm = (global::Avalonia.Controls.DataGridHierarchical.IHierarchicalModel)grid.HierarchicalModel!;
+		var flat = (System.Collections.IList)grid.ItemsSource!;
 		// Expand the first assembly so there are plenty of rows and a real "middle".
-		var firstAsm = (SharpTreeNode)hm.Flattened[0].Item;
+		var firstAsm = (SharpTreeNode)flat[0]!;
 		firstAsm.EnsureLazyChildren();
 		firstAsm.IsExpanded = true;
 		Dispatcher.UIThread.RunJobs();
-		await Waiters.WaitForAsync(() => hm.Flattened.Count >= 7);
+		await Waiters.WaitForAsync(() => flat.Count >= 7);
 
-		var start = (SharpTreeNode)hm.Flattened[3].Item;
-		var below = (SharpTreeNode)hm.Flattened[4].Item;
+		var start = (SharpTreeNode)flat[3]!;
+		var below = (SharpTreeNode)flat[4]!;
 		model.SelectNode(start);
 		await Waiters.WaitForAsync(() => ReferenceEquals(model.SelectedItem, start));
 
@@ -999,7 +998,7 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var model = vm.AssemblyTreeModel;
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.Focus();
 		Dispatcher.UIThread.RunJobs();
 
@@ -1099,40 +1098,27 @@ public class AssemblyTreeTests
 	}
 
 	[AvaloniaTest]
-	public async Task Setting_SharpTreeNode_IsExpanded_Expands_Row_In_Grid()
+	public async Task Setting_SharpTreeNode_IsExpanded_Reveals_Children_In_The_Tree()
 	{
-		// ProDataGrid's HierarchicalOptions.IsExpandedPropertyPath wires SharpTreeNode.IsExpanded
-		// to the grid's wrapper expansion state via reflection + INotifyPropertyChanged. Setting
-		// IsExpanded on the model must propagate to the grid wrapper without any manual
-		// subscription bookkeeping.
-
-		// Arrange — boot, wait for assemblies, locate the realised DataGrid and the
-		// HierarchicalModel wrapper for the assembly node.
+		// SharpTreeView binds the TreeFlattener directly, so toggling SharpTreeNode.IsExpanded
+		// reveals the node's children in the flattened row list with no wrapper bookkeeping.
 		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
-		var hm = (global::Avalonia.Controls.DataGridHierarchical.IHierarchicalModel)grid.HierarchicalModel!;
-		var hNode = hm.FindNode(assemblyNode);
-		hNode.Should().NotBeNull();
-		hNode!.IsExpanded.Should().BeFalse("baseline: top-level assembly rows start collapsed");
+		assemblyNode.IsExpanded.Should().BeFalse("baseline: top-level assembly rows start collapsed");
+		var flat = (System.Collections.IList)grid.ItemsSource!;
+		int before = flat.Count;
 
 		// Act — production-shaped action: just toggle the model property.
 		assemblyNode.Expand();
-
-		// Drain the dispatcher so the wiring (PropertyChanged → hm.Expand) settles.
-		for (int i = 0; i < 5; i++)
-		{
-			Dispatcher.UIThread.RunJobs();
-			await Task.Delay(20);
-		}
 		TestCapture.Step("assembly-row-expanded");
 
-		// Assert — the grid wrapper now reports IsExpanded == true.
-		hm.FindNode(assemblyNode)!.IsExpanded.Should().BeTrue(
-			"setting SharpTreeNode.IsExpanded must propagate to the HierarchicalModel wrapper");
+		// Assert — the node's children now appear in the flattened tree.
+		await Waiters.WaitForAsync(() => flat.Count > before,
+			description: "setting SharpTreeNode.IsExpanded must reveal its children in the flattened tree");
 	}
 
 	[AvaloniaTest]
@@ -1568,7 +1554,7 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.UpdateLayout();
 
 		var topLevelCount = vm.AssemblyTreeModel.Root!.Children.Count;
@@ -1615,7 +1601,7 @@ public class AssemblyTreeTests
 		var (window, vm) = await TestHarness.BootAsync(3);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 		grid.UpdateLayout();
 
 		var topLevelCount = vm.AssemblyTreeModel.Root!.Children.Count;
@@ -1629,13 +1615,16 @@ public class AssemblyTreeTests
 		TestCapture.Step("all-rows-selected");
 
 		// Act — plain left-click on the second visible row.
-		var targetRow = grid.GetVisualDescendants().OfType<DataGridRow>()
+		var targetRow = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
 			.OrderBy(r => r.TranslatePoint(new Point(0, 0), grid)?.Y ?? double.MaxValue)
 			.Skip(1).First();
-		var targetNode = (targetRow.DataContext as HierarchicalNode)?.Item as SharpTreeNode;
+		var targetNode = targetRow.DataContext as SharpTreeNode;
 		Assert.That(targetNode, Is.Not.Null, "the clicked row must wrap a tree node");
+		// Tree rows stretch to content width (with horizontal scroll), so a row can be wider than
+		// the grid viewport. Click within the visible viewport, not at the (off-screen) row centre.
+		var clickX = System.Math.Min(targetRow.Bounds.Width, grid.Bounds.Width) / 2;
 		var rowCentre = targetRow.TranslatePoint(
-			new Point(targetRow.Bounds.Width / 2, targetRow.Bounds.Height / 2), window)!.Value;
+			new Point(clickX, targetRow.Bounds.Height / 2), window)!.Value;
 		HeadlessWindowExtensions.MouseDown(window, rowCentre, MouseButton.Left);
 		HeadlessWindowExtensions.MouseUp(window, rowCentre, MouseButton.Left);
 		Dispatcher.UIThread.RunJobs();

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesGridVerification.cs

@@ -16,13 +16,16 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System.Collections;
+using System.Linq;
 using System.Threading.Tasks;
 
-using Avalonia.Controls;
 using Avalonia.Headless.NUnit;
 
 using AwesomeAssertions;
 
+using ICSharpCode.ILSpyX.TreeView;
+
 using ILSpy;
 using ILSpy.AppEnv;
 using ILSpy.AssemblyTree;
@@ -36,42 +39,53 @@ namespace ICSharpCode.ILSpy.Tests;
 public class UseNestedNamespaceNodesGridVerification
 {
 	[AvaloniaTest]
-	public async Task Toggling_UseNestedNamespaceNodes_Triggers_BindTree_So_The_Grid_Sees_The_New_Shape()
+	public async Task Toggling_UseNestedNamespaceNodes_Reshapes_The_Visible_Tree_In_Place()
 	{
-		// The pane caches a snapshot of each expanded node's children via the
-		// HierarchicalOptions.ChildrenSelector — mid-expand mutations to AssemblyTreeNode.
-		// Children aren't observed by the live grid. The fix raises AssemblyTreeModel.Root
-		// PropertyChanged so AssemblyListPane.BindTree fires and replaces the
-		// HierarchicalModel; this test verifies that re-bind happens.
+		// Toggling UseNestedNamespaceNodes rebuilds each loaded assembly's namespace subtree.
+		// SharpTreeView's flattener observes node.Children mutations live, so the rebuilt shape
+		// surfaces in the visible row list without any model re-bind (the ProDataGrid snapshot
+		// problem that used to need a HierarchicalModel rebind is gone). This pins that the
+		// flat-mode -> nested-mode switch is reflected in the SharpTreeView's ItemsSource.
 
 		var settings = AppComposition.Current.GetExport<SettingsService>().DisplaySettings;
 		settings.UseNestedNamespaceNodes = false;
 
 		var (window, vm) = await TestHarness.BootAsync(3);
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = pane.FindControl<DataGrid>("TreeGrid")!;
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
-		// Expand an assembly to force the ChildrenSelector to fire and cache its snapshot.
+		// Expand an assembly so its namespace children become visible rows.
 		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>("System.Linq");
 		assemblyNode.IsExpanded = true;
-		await Waiters.WaitForAsync(() => grid.HierarchicalModel != null);
+		await Waiters.WaitForAsync(() => assemblyNode.Children.OfType<NamespaceTreeNode>().Any());
 		TestCapture.Step("system-linq-expanded-flat");
 
+		var flat = (IList)grid.ItemsSource!;
+		bool VisibleNamespace(string name) => flat.Cast<SharpTreeNode>()
+			.OfType<NamespaceTreeNode>()
+			.Any(n => string.Equals(n.Text?.ToString(), name, System.StringComparison.Ordinal));
+
+		// In flat mode the full dotted namespace is a single visible row.
+		await Waiters.WaitForAsync(() => VisibleNamespace("System.Collections.Generic"),
+			description: "flat mode shows the dotted namespace as one row");
+
 		try
 		{
-			var modelBefore = grid.HierarchicalModel;
-			((object?)modelBefore).Should().NotBeNull("baseline: the grid must have a HierarchicalModel before the toggle");
-
 			settings.UseNestedNamespaceNodes = true;
+			assemblyNode.IsExpanded = true;
+			TestCapture.Step("nested-namespaces-rebuilt");
 
+			// In nested mode the leaf segment ("Generic") becomes its own row nested under
+			// "System" -> "Collections"; the live flattener must surface that without a re-bind.
 			await Waiters.WaitForAsync(
-				() => !ReferenceEquals(grid.HierarchicalModel, modelBefore),
-				System.TimeSpan.FromSeconds(5));
-			TestCapture.Step("nested-namespaces-rebound");
+				() => assemblyNode.Children.OfType<NamespaceTreeNode>()
+						.Any(n => string.Equals(n.Text?.ToString(), "System", System.StringComparison.Ordinal)),
+				System.TimeSpan.FromSeconds(5),
+				"toggling UseNestedNamespaceNodes must rebuild the namespace subtree into nested nodes");
 
-			grid.HierarchicalModel.Should().NotBeSameAs(modelBefore,
-				"toggling UseNestedNamespaceNodes must force AssemblyListPane.BindTree, "
-				+ "replacing the HierarchicalModel with one that reads the rebuilt children");
+			VisibleNamespace("System").Should().BeTrue(
+				"the rebuilt nested 'System' namespace node must appear in the SharpTreeView's visible rows "
+				+ "without any model re-bind (the live flattener observes the child mutations directly)");
 		}
 		finally
 		{

ILSpy.Tests/AssemblyList/UseNestedNamespaceNodesScreenshot.cs

@@ -54,7 +54,7 @@ public class UseNestedNamespaceNodesScreenshot
 		await Waiters.WaitForAsync(() =>
 			assemblyNode.Children.Count > 0
 				&& assemblyNode.Children[assemblyNode.Children.Count - 1] is NamespaceTreeNode);
-		// Re-expand — BindTree replaces the HierarchicalModel and resets visible expansion.
+		// Re-expand — the namespace subtree was rebuilt, so make it visible again.
 		assemblyNode.IsExpanded = true;
 		Dispatcher.UIThread.RunJobs();
 

ILSpy.Tests/Diagnostics/StartupPerfTests.cs

@@ -121,13 +121,13 @@ public class StartupPerfTests
 				+ $"({swLoad.ElapsedMilliseconds / (double)allAssemblies.Length:0.##} ms/asm)");
 
 			// Act 4 — the assembly-tree pane should still be responsive. Measure how long it
-			// takes the AssemblyListPane to produce a fresh DataGrid descendant from this point
+			// takes the AssemblyListPane to produce a fresh SharpTreeView descendant from this point
 			// (proxy for "tree is interactive").
 			var pane = await window.WaitForComponent<AssemblyListPane>();
 			var swGrid = Stopwatch.StartNew();
-			var grid = await pane.WaitForComponent<DataGrid>();
+			var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 			swGrid.Stop();
-			TestContext.Out.WriteLine($"DataGrid descendant available: {swGrid.ElapsedMilliseconds} ms");
+			TestContext.Out.WriteLine($"SharpTreeView descendant available: {swGrid.ElapsedMilliseconds} ms");
 
 			// Sanity assertions — large enough to never trip on slow machines, but tight
 			// enough to flag a 10× regression.

ILSpy.Tests/Input/HeadlessMmbPointerTests.cs

@@ -21,7 +21,6 @@ using System.Threading.Tasks;
 
 using Avalonia;
 using Avalonia.Controls;
-using Avalonia.Controls.DataGridHierarchical;
 using Avalonia.Headless;
 using Avalonia.Headless.NUnit;
 using Avalonia.Input;
@@ -60,16 +59,14 @@ public class HeadlessMmbPointerTests
 		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
-		// Any realised DataGridRow whose DataContext wraps an ILSpyTreeNode is a valid click
+		// Any realised SharpTreeViewItem whose DataContext is an ILSpyTreeNode is a valid click
 		// target — the exact node doesn't matter, only that the gesture pipeline fires.
-		// ProDataGrid wraps each row's data in a HierarchicalNode (`{ Item: ILSpyTreeNode }`),
-		// so we walk that surface rather than comparing against the tree node directly.
-		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<DataGridRow>()
-			.Any(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode }));
-		var row = grid.GetVisualDescendants().OfType<DataGridRow>()
-			.First(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode });
+		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.Any(r => r.DataContext is ILSpyTreeNode));
+		var row = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.First(r => r.DataContext is ILSpyTreeNode);
 
 		// Snapshot the tab count BEFORE the gesture so we can assert a strict +1 after.
 		var documents = ((ILSpyDockFactory)vm.DockWorkspace.Factory).Documents!;
@@ -78,11 +75,13 @@ public class HeadlessMmbPointerTests
 		// Translate the row's centre into TopLevel coordinates so MouseDown lands inside it.
 		var topLevel = TopLevel.GetTopLevel(row)!;
 		var rowBounds = row.Bounds;
-		var centreInRow = new Point(rowBounds.Width / 2, rowBounds.Height / 2);
+		// Tree rows stretch to content width (with horizontal scroll), so clamp the click X to the
+		// visible grid viewport — the row centre can sit off-screen past the grid's right edge.
+		var centreInRow = new Point(System.Math.Min(rowBounds.Width, grid.Bounds.Width) / 2, rowBounds.Height / 2);
 		var centreInTopLevel = row.TranslatePoint(centreInRow, topLevel)
 			?? throw new System.InvalidOperationException("Row not in TopLevel visual tree");
 
-		// Real pointer event — exercises bubble + handledEventsToo routing through ProDataGrid.
+		// Real pointer event — exercises bubble + handledEventsToo routing through SharpTreeView.
 		topLevel.MouseDown(centreInTopLevel, MouseButton.Middle);
 		topLevel.MouseUp(centreInTopLevel, MouseButton.Middle);
 
@@ -95,8 +94,8 @@ public class HeadlessMmbPointerTests
 	[AvaloniaTest]
 	public async Task Ctrl_LMB_On_An_Assembly_Tree_Row_Does_Not_Open_A_New_Tab()
 	{
-		// Ctrl+LMB previously did double duty: ProDataGrid's Extended-selection mode treats
-		// it as "toggle this row in the multi-selection", and our OnTreeGridPointerPressed
+		// Ctrl+LMB previously did double duty: the tree's Extended-selection mode treats
+		// it as "toggle this row in the multi-selection", and our OnTreePointerPressed
 		// also treated it as "open in new tab". Both fired on a single click, so users
 		// trying to extend a multi-selection ended up spawning unwanted tabs. The intended
 		// gesture for "open in new tab" is MMB (matches WPF's DecompileInNewViewCommand
@@ -109,19 +108,21 @@ public class HeadlessMmbPointerTests
 		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
-		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<DataGridRow>()
-			.Any(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode }));
-		var row = grid.GetVisualDescendants().OfType<DataGridRow>()
-			.First(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode });
+		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.Any(r => r.DataContext is ILSpyTreeNode));
+		var row = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.First(r => r.DataContext is ILSpyTreeNode);
 
 		var documents = ((ILSpyDockFactory)vm.DockWorkspace.Factory).Documents!;
 		int before = documents.VisibleDockables?.Count ?? 0;
 
 		var topLevel = TopLevel.GetTopLevel(row)!;
 		var rowBounds = row.Bounds;
-		var centreInRow = new Point(rowBounds.Width / 2, rowBounds.Height / 2);
+		// Tree rows stretch to content width (with horizontal scroll), so clamp the click X to the
+		// visible grid viewport — the row centre can sit off-screen past the grid's right edge.
+		var centreInRow = new Point(System.Math.Min(rowBounds.Width, grid.Bounds.Width) / 2, rowBounds.Height / 2);
 		var centreInTopLevel = row.TranslatePoint(centreInRow, topLevel)
 			?? throw new System.InvalidOperationException("Row not in TopLevel visual tree");
 
@@ -133,7 +134,7 @@ public class HeadlessMmbPointerTests
 		global::Avalonia.Threading.Dispatcher.UIThread.RunJobs();
 
 		(documents.VisibleDockables?.Count ?? 0).Should().Be(before,
-			"Ctrl+LMB must be reserved for ProDataGrid's multi-selection toggle — only MMB opens a new tab");
+			"Ctrl+LMB must be reserved for the tree's multi-selection toggle — only MMB opens a new tab");
 	}
 
 	[AvaloniaTest]
@@ -151,22 +152,24 @@ public class HeadlessMmbPointerTests
 		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
 
 		var pane = await window.WaitForComponent<AssemblyListPane>();
-		var grid = await pane.WaitForComponent<DataGrid>();
+		var grid = await pane.WaitForComponent<global::ILSpy.Controls.TreeView.SharpTreeView>();
 
-		// Any realised DataGridRow whose data wraps an ILSpyTreeNode is a fine target; the
+		// Any realised SharpTreeViewItem whose data is an ILSpyTreeNode is a fine target; the
 		// non-leaf assembly node row (depth 0) is always present and never confused with a
 		// method row whose tree-toggle area is null.
-		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<DataGridRow>()
-			.Any(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode }));
-		var row = grid.GetVisualDescendants().OfType<DataGridRow>()
-			.First(r => r.DataContext is HierarchicalNode { Item: ILSpyTreeNode });
+		await Waiters.WaitForAsync(() => grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.Any(r => r.DataContext is ILSpyTreeNode));
+		var row = grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>()
+			.First(r => r.DataContext is ILSpyTreeNode);
 
 		var documents = ((ILSpyDockFactory)vm.DockWorkspace.Factory).Documents!;
 		int before = documents.VisibleDockables?.Count ?? 0;
 
 		var topLevel = TopLevel.GetTopLevel(row)!;
 		var rowBounds = row.Bounds;
-		var centreInRow = new Point(rowBounds.Width / 2, rowBounds.Height / 2);
+		// Tree rows stretch to content width (with horizontal scroll), so clamp the click X to the
+		// visible grid viewport — the row centre can sit off-screen past the grid's right edge.
+		var centreInRow = new Point(System.Math.Min(rowBounds.Width, grid.Bounds.Width) / 2, rowBounds.Height / 2);
 		var centreInTopLevel = row.TranslatePoint(centreInRow, topLevel)
 			?? throw new System.InvalidOperationException("Row not in TopLevel visual tree");
 

ILSpy.Tests/Options/ApiVisibilityFilterTests.cs

@@ -21,7 +21,6 @@ using System.Threading.Tasks;
 using System.Xml.Linq;
 
 using Avalonia.Controls;
-using Avalonia.Controls.DataGridHierarchical;
 using Avalonia.Controls.Primitives;
 using Avalonia.Headless.NUnit;
 using Avalonia.VisualTree;

ILSpy.Tests/TreeNodeAssertions.cs

@@ -79,11 +79,11 @@ public class TreeNodeBeAssertions
 		}
 
 		var scrollViewer = grid.GetVisualDescendants().OfType<ScrollViewer>().FirstOrDefault()
-			?? throw new InvalidOperationException("DataGrid has no ScrollViewer in its visual tree.");
+			?? throw new InvalidOperationException("SharpTreeView has no ScrollViewer in its visual tree.");
 
 		// ScrollIntoView is posted at Background priority — give it time to realise the row.
 		var deadline = DateTime.UtcNow + ScrollPollTimeout;
-		DataGridRow? row = null;
+		global::ILSpy.Controls.TreeView.SharpTreeViewItem? row = null;
 		bool ok = false;
 		while (DateTime.UtcNow < deadline)
 		{
@@ -104,7 +104,7 @@ public class TreeNodeBeAssertions
 		return new AndConstraint<TreeNodeBeAssertions>(this);
 	}
 
-	static DataGrid? TryFindGrid(out string? failure)
+	static global::ILSpy.Controls.TreeView.SharpTreeView? TryFindGrid(out string? failure)
 	{
 		var window = FindActiveWindow();
 		if (window is null)
@@ -120,10 +120,10 @@ public class TreeNodeBeAssertions
 			return null;
 		}
 
-		var grid = pane.GetVisualDescendants().OfType<DataGrid>().FirstOrDefault(g => g.Name == "TreeGrid");
+		var grid = pane.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeView>().FirstOrDefault();
 		if (grid is null)
 		{
-			failure = "TreeGrid DataGrid not found inside AssemblyListPane";
+			failure = "SharpTreeView not found inside AssemblyListPane";
 			return null;
 		}
 
@@ -145,22 +145,17 @@ public class TreeNodeBeAssertions
 		}
 	}
 
-	static DataGridRow? FindRow(DataGrid grid, SharpTreeNode target)
+	static global::ILSpy.Controls.TreeView.SharpTreeViewItem? FindRow(global::ILSpy.Controls.TreeView.SharpTreeView grid, SharpTreeNode target)
 	{
-		foreach (var row in grid.GetVisualDescendants().OfType<DataGridRow>())
+		foreach (var row in grid.GetVisualDescendants().OfType<global::ILSpy.Controls.TreeView.SharpTreeViewItem>())
 		{
-			var ctx = row.DataContext;
-			if (ReferenceEquals(ctx, target))
-				return row;
-			// HierarchicalNode wraps the underlying item via .Item.
-			var itemProp = ctx?.GetType().GetProperty("Item");
-			if (itemProp is not null && ReferenceEquals(itemProp.GetValue(ctx), target))
+			if (ReferenceEquals(row.DataContext, target))
 				return row;
 		}
 		return null;
 	}
 
-	static bool IsInViewport(DataGridRow row, ScrollViewer scrollViewer)
+	static bool IsInViewport(global::ILSpy.Controls.TreeView.SharpTreeViewItem row, ScrollViewer scrollViewer)
 	{
 		var topLeft = row.TranslatePoint(new Point(0, 0), scrollViewer);
 		if (topLeft is null)
@@ -170,7 +165,7 @@ public class TreeNodeBeAssertions
 		return top >= 0 && bottom <= scrollViewer.Viewport.Height + 0.5;
 	}
 
-	static bool IsRoughlyCentered(DataGridRow row, ScrollViewer scrollViewer)
+	static bool IsRoughlyCentered(global::ILSpy.Controls.TreeView.SharpTreeViewItem row, ScrollViewer scrollViewer)
 	{
 		var topLeft = row.TranslatePoint(new Point(0, 0), scrollViewer);
 		if (topLeft is null)
@@ -183,10 +178,10 @@ public class TreeNodeBeAssertions
 		return Math.Abs(rowMid - viewportMid) <= row.Bounds.Height || nearTopClamp || nearBottomClamp;
 	}
 
-	string BuildState(ScrollViewer scrollViewer, DataGridRow? row)
+	string BuildState(ScrollViewer scrollViewer, global::ILSpy.Controls.TreeView.SharpTreeViewItem? row)
 	{
 		if (row is null)
-			return $"but no DataGridRow has been realised for it (offset={scrollViewer.Offset}, viewport={scrollViewer.Viewport}, extent={scrollViewer.Extent})";
+			return $"but no SharpTreeViewItem has been realised for it (offset={scrollViewer.Offset}, viewport={scrollViewer.Viewport}, extent={scrollViewer.Extent})";
 		var topLeft = row.TranslatePoint(new Point(0, 0), scrollViewer);
 		var top = topLeft?.Y;
 		var bottom = top + row.Bounds.Height;

ILSpy/Controls/TreeView/SharpTreeView.axaml

@@ -59,7 +59,8 @@
 					<StackPanel Orientation="Horizontal" Height="20">
 						<!-- Indent spacer = (Level - 1) * step (top level sits flush left). -->
 						<Border Width="{Binding Level, Converter={x:Static tv:TreeIndentConverter.Instance}}" />
-						<ToggleButton Theme="{StaticResource TreeExpanderToggleTheme}"
+						<ToggleButton Name="PART_Expander"
+						              Theme="{StaticResource TreeExpanderToggleTheme}"
 						              VerticalAlignment="Center"
 						              IsChecked="{Binding IsExpanded, Mode=TwoWay}"
 						              IsVisible="{Binding ShowExpander}" />

ILSpy/Controls/TreeView/SharpTreeView.cs

@@ -72,6 +72,9 @@ namespace ILSpy.Controls.TreeView
 
 		public SharpTreeView()
 		{
+			// Take keyboard focus directly so gestures like Ctrl+A work the moment the user tabs
+			// or clicks into the pane, before any row becomes the current item.
+			Focusable = true;
 			SelectionChanged += OnSelectionChanged;
 			DoubleTapped += OnDoubleTapped;
 			// Drag-drop is handled generically here and delegated to SharpTreeNode.CanDrop/Drop.
@@ -204,6 +207,14 @@ namespace ILSpy.Controls.TreeView
 				Dispatcher.UIThread.Post(() => (ContainerFromItem(node))?.Focus(), DispatcherPriority.Loaded);
 		}
 
+		/// <summary>Moves the (single) selection to <paramref name="node"/> and focuses it.
+		/// Used by keyboard navigation where selection must follow the caret.</summary>
+		void SelectAndFocus(SharpTreeNode node)
+		{
+			SelectedItem = node;
+			FocusNode(node);
+		}
+
 		public void ScrollIntoNodeView(SharpTreeNode node)
 		{
 			ArgumentNullException.ThrowIfNull(node);
@@ -216,6 +227,16 @@ namespace ILSpy.Controls.TreeView
 
 		protected override void OnKeyDown(KeyEventArgs e)
 		{
+			// Ctrl+A select-all must work on the first press even before a current item is
+			// established (right after the tree gains focus). The base ListBox only selects all
+			// when a focused item lives inside it, so drive it explicitly here.
+			if (e.Key == Key.A && e.KeyModifiers == KeyModifiers.Control
+				&& SelectionMode.HasFlag(SelectionMode.Multiple))
+			{
+				SelectAll();
+				e.Handled = true;
+				return;
+			}
 			var node = (e.Source as Visual)?.FindAncestorOfType<SharpTreeViewItem>(includeSelf: true)?.Node
 				?? SelectedItem as SharpTreeNode;
 			if (node != null && e.KeyModifiers == KeyModifiers.None)
@@ -226,7 +247,7 @@ namespace ILSpy.Controls.TreeView
 						if (node.IsExpanded)
 							node.IsExpanded = false;
 						else if (node.Parent != null && !node.Parent.IsRoot)
-							FocusNode(node.Parent);
+							SelectAndFocus(node.Parent);
 						else
 							break;
 						e.Handled = true;
@@ -235,7 +256,7 @@ namespace ILSpy.Controls.TreeView
 						if (!node.IsExpanded && node.ShowExpander)
 							node.IsExpanded = true;
 						else if (node.Children.Count > 0)
-							FocusNode(node.Children.First(c => c.IsVisible));
+							SelectAndFocus(node.Children.First(c => c.IsVisible));
 						else
 							break;
 						e.Handled = true;