Add Left/Right keyboard expand-collapse and parent/child nav

Standard tree keyboard navigation in the assembly list: Left collapses an expanded node or moves to the parent when already collapsed; Right expands a closed node or steps into its first child. Operates on the primary selection via the hierarchical model (FindNode/Expand/Collapse) and SelectNode; the parent step is gated on the parent being a visible row so it never selects the hidden root.

ILSpy.Tests/AssemblyList/AssemblyTreeTests.cs

@@ -856,6 +856,44 @@ public class AssemblyTreeTests
 			description: "a second Delete must unload another assembly; the selection didn't desync");
 	}
 
+	[AvaloniaTest]
+	public async Task Left_And_Right_Keys_Collapse_Expand_And_Navigate_The_Tree()
+	{
+		// Standard tree keyboard nav: Right expands a collapsed node then steps into its first
+		// child; Left collapses an expanded node, or moves to the parent when already collapsed.
+		var (window, vm) = await TestHarness.BootAsync(3);
+		var model = vm.AssemblyTreeModel;
+		var pane = await window.WaitForComponent<AssemblyListPane>();
+		var grid = await pane.WaitForComponent<DataGrid>();
+		grid.Focus();
+		Dispatcher.UIThread.RunJobs();
+
+		var assembly = model.FindNode<AssemblyTreeNode>("System.Linq");
+		assembly.EnsureLazyChildren();
+		model.SelectNode(assembly);
+		await Waiters.WaitForAsync(() => ReferenceEquals(model.SelectedItem, assembly));
+		assembly.IsExpanded.Should().BeFalse("precondition: the assembly starts collapsed");
+
+		// Right expands.
+		window.KeyPress(Key.Right, RawInputModifiers.None, PhysicalKey.ArrowRight, null);
+		await Waiters.WaitForAsync(() => assembly.IsExpanded, description: "Right expands the collapsed node");
+
+		// Right again steps into the first child.
+		window.KeyPress(Key.Right, RawInputModifiers.None, PhysicalKey.ArrowRight, null);
+		await Waiters.WaitForAsync(
+			() => model.SelectedItem is { } s && !ReferenceEquals(s, assembly) && ReferenceEquals(s.Parent, assembly),
+			description: "Right on an expanded node selects its first child");
+
+		// Left on the (collapsed/leaf) child moves selection back to the parent.
+		window.KeyPress(Key.Left, RawInputModifiers.None, PhysicalKey.ArrowLeft, null);
+		await Waiters.WaitForAsync(() => ReferenceEquals(model.SelectedItem, assembly),
+			description: "Left on a collapsed child selects the parent");
+
+		// Left on the expanded parent collapses it.
+		window.KeyPress(Key.Left, RawInputModifiers.None, PhysicalKey.ArrowLeft, null);
+		await Waiters.WaitForAsync(() => !assembly.IsExpanded, description: "Left collapses the expanded node");
+	}
+
 	[AvaloniaTest]
 	public async Task Clear_Assembly_List_Command_Empties_The_Active_List()
 	{

ILSpy/AssemblyTree/AssemblyListPane.axaml.cs

@@ -310,6 +310,34 @@ namespace ILSpy.AssemblyTree
 				Dispatcher.UIThread.Post(() => ReselectAfterDelete(reselectIndex), DispatcherPriority.Background);
 				return;
 			}
+			if (e.Key is Key.Left or Key.Right && e.KeyModifiers == KeyModifiers.None
+				&& model.SelectedItem is { } current
+				&& TreeGrid.HierarchicalModel is IHierarchicalModel hmNav
+				&& hmNav.FindNode(current) is { } currentNode)
+			{
+				if (e.Key == Key.Left)
+				{
+					// Collapse if open; otherwise step out to the parent (unless it's the hidden root).
+					if (currentNode.IsExpanded)
+						hmNav.Collapse(currentNode);
+					else if (current.Parent is { } parent && hmNav.FindNode(parent) is not null)
+						model.SelectNode(parent);
+					else
+						return;
+				}
+				else
+				{
+					// Expand if closed and has children; otherwise step into the first child.
+					if (!currentNode.IsExpanded && !currentNode.IsLeaf)
+						hmNav.Expand(currentNode);
+					else if (currentNode.IsExpanded && currentNode.Children.Count > 0)
+						model.SelectNode(currentNode.Children[0].Item as SharpTreeNode);
+					else
+						return;
+				}
+				e.Handled = true;
+				return;
+			}
 			if (e.Key == Key.R && e.KeyModifiers == KeyModifiers.Control)
 			{
 				var members = model.SelectedItems.OfType<IMemberTreeNode>()