Make the Window menu actually show a default-hidden tool pane

Window > Debug Steps did nothing. Two bugs in ToolPaneMenuItem:
the getter treated a pane with a null Owner (one hidden by default and
never placed in the layout) as visible, so the menu showed it checked
and toggling tried to hide it; and the show branch used
factory.RestoreDockable, which only un-hides a previously-shown pane and
is a no-op for one that was never in the layout.

Report visibility from real layout membership, and show via
ShowToolPane, which materialises the pane and (re)creates its home dock.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy.Tests/Views/DebugStepsTests.cs

@@ -133,6 +133,28 @@ public class DebugStepsTests
 		languageService.Languages.Should().Contain(l => l.Name == "Typed IL");
 		return Task.CompletedTask;
 	}
+	[AvaloniaTest]
+	public async Task Window_Menu_Toggle_Surfaces_The_Default_Hidden_Debug_Steps_Pane()
+	{
+		// Repro of "Window > Debug Steps does nothing": the menu toggles ToolPaneMenuItem.IsPaneVisible,
+		// which used factory.RestoreDockable — a no-op for a pane that is hidden by default (never
+		// placed in the layout, so there's nothing to restore). Toggling it on must actually surface
+		// the pane (ShowToolPane materialises it and creates its home dock).
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+
+		var menuItem = vm.DockWorkspace.ToolPaneMenuItems
+			.Single(p => p.Title == "Debug Steps");
+		menuItem.IsPaneVisible.Should().BeFalse("Debug Steps is hidden by default");
+
+		menuItem.IsPaneVisible = true;
+
+		menuItem.IsPaneVisible.Should().BeTrue(
+			"toggling Window > Debug Steps on must make the pane visible in the layout");
+	}
+
 }
 
 #endif

ILSpy/ViewModels/ToolPaneMenuItem.cs

@@ -39,12 +39,27 @@ namespace ILSpy.ViewModels
 		public string? Title => pane.Title;
 
 		public bool IsPaneVisible {
-			get => pane.Owner is not IRootDock;
+			// Visible only when the pane currently sits in a real (non-root) dock's visible
+			// dockables. A pane that is hidden by default and never shown has a null Owner — the
+			// old `Owner is not IRootDock` test reported that as visible, so the Window-menu toggle
+			// thought it was already open and "closed" it (a no-op) instead of showing it.
+			get => pane.Owner is IDock owner
+				&& owner is not IRootDock
+				&& owner.VisibleDockables?.Contains(pane) == true;
 			set {
 				if (value == IsPaneVisible)
 					return;
 				if (value)
+				{
+					// RestoreDockable only un-hides a dockable that was previously shown and then
+					// hidden; a pane that is hidden by default (e.g. Debug Steps) was never placed in
+					// the layout, so there is nothing to restore. ShowToolPane materialises the pane
+					// AND (re)creates its home dock, so it works in both cases.
+					if (factory is Docking.ILSpyDockFactory ilspyFactory)
+						ilspyFactory.ShowToolPane(pane.Id);
+					else
 						factory.RestoreDockable(pane);
+				}
 				else
 					factory.HideDockable(pane);
 				OnPropertyChanged();