Disable the tab Close menu entry when the tab can't be closed

DockWorkspace keeps the last remaining document un-closeable (CanClose
false) so the document area can't be emptied, and Dock's own close
button already honours that. The context-menu Close entry did not, so a
right-click still offered to close the final tab. Bind its IsEnabled to
the tab's CanClose, which is observable, so it tracks tabs opening and
closing.

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

ILSpy.Tests/Docking/DocumentTabContextMenuTests.cs

@@ -79,6 +79,32 @@ public class DocumentTabContextMenuTests
 			.Should().BeSameAs(a.CloseAllCommand);
 	}
 
+	[AvaloniaTest]
+	public async Task Close_Entry_Is_Disabled_When_The_Tab_Cannot_Be_Closed()
+	{
+		// The last remaining document is made un-closeable (DockWorkspace.UpdateLastDocumentCanClose
+		// flips CanClose to false so the user can't empty the document area). The context-menu Close
+		// entry must honour that the same way Dock's own close button does, so it binds IsEnabled to
+		// the tab's CanClose.
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+		var (a, _, _) = OpenThreeTabs(vm.DockWorkspace);
+
+		var menu = global::ILSpy.Themes.PreviewTabContextMenuBehavior.BuildDocumentContextMenu(a);
+		var close = menu.ItemsSource!.OfType<global::Avalonia.Controls.MenuItem>()
+			.Single(m => Equals(m.Header, ICSharpCode.ILSpy.Properties.Resources.Close));
+
+		a.CanClose = true;
+		Dispatcher.UIThread.RunJobs();
+		close.IsEnabled.Should().BeTrue("a closeable tab's Close entry is enabled");
+
+		a.CanClose = false;
+		Dispatcher.UIThread.RunJobs();
+		close.IsEnabled.Should().BeFalse("an un-closeable tab's Close entry is disabled");
+	}
+
 	[AvaloniaTest]
 	public async Task Close_All_But_This_Leaves_Only_The_Target_Tab()
 	{

ILSpy/Themes/PreviewTabContextMenuBehavior.cs

@@ -20,6 +20,8 @@ using System.ComponentModel;
 
 using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Data;
+using Avalonia.Input;
 
 using Dock.Avalonia.Controls;
 
@@ -90,6 +92,12 @@ namespace ILSpy.Themes
 		internal static ContextMenu BuildDocumentContextMenu(ContentTabPage tab)
 		{
 			var close = new MenuItem { Header = Resources.Close, Command = tab.CloseCommand };
+			// The last remaining document is made un-closeable (DockWorkspace keeps CanClose false so
+			// the document area can't be emptied). Mirror that on the Close entry, the way Dock's own
+			// close button does -- CanClose is observable (DockableBase : ReactiveBase), so this stays
+			// in sync as tabs open and close.
+			close.Bind(InputElement.IsEnabledProperty,
+				new Binding(nameof(ContentTabPage.CanClose)) { Source = tab });
 			var closeAllButThis = new MenuItem { Header = Resources.CloseAllButThisTab, Command = tab.CloseAllButThisCommand };
 			var closeAll = new MenuItem { Header = Resources.CloseAllTabs, Command = tab.CloseAllCommand };