Make MEF-menu separators visible

The InitMainMenu build path already inserts Separators between
MenuCategory groups (Open / Save / Remove / Exit in File, View /
Navigation / Options in View, etc.). The Simple-theme default style
renders them as a 1px line at very low contrast — barely visible
against the menu background, which is why "no groups in the menus"
was a reasonable read.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy.Tests/Docking/PreviewTabPromotionTests.cs

@@ -490,6 +490,36 @@ public class PreviewTabPromotionTests
 			"the already-pinned tab stays pinned");
 	}
 
+	[AvaloniaTest]
+	public async Task File_Menu_Separators_Have_A_Visible_Background()
+	{
+		// Regression for the "no groups in the menus" report: separators were inserted at
+		// MenuCategory boundaries (Open / Save / Remove / Exit in File menu, etc.) but the
+		// Simple-theme default rendered them nearly invisible against the menu chrome.
+		// App.axaml applies a Style for MenuItem Separator that gives them a visible
+		// brush + height. This test pins that style at the rendered-Separator level.
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+
+		var fileMenu = window.GetVisualDescendants().OfType<global::Avalonia.Controls.MenuItem>()
+			.Single(m => (m.Tag as string) == "_File");
+		fileMenu.Open();
+		global::Avalonia.Threading.Dispatcher.UIThread.RunJobs();
+		await Waiters.WaitForAsync(() => fileMenu.Items.Count > 0, System.TimeSpan.FromSeconds(5));
+
+		var separators = fileMenu.Items.OfType<global::Avalonia.Controls.Separator>().ToList();
+		separators.Should().HaveCountGreaterThanOrEqualTo(2,
+			"File menu's MenuCategory groups (Open / Save / Remove / Exit) must produce at least two separators");
+
+		foreach (var sep in separators)
+		{
+			sep.Background.Should().NotBeNull(
+				"the App.axaml MenuItem Separator style must set a visible Background brush");
+		}
+	}
+
 	[AvaloniaTest]
 	public async Task Pin_Button_Inherits_The_Close_Button_Theme()
 	{

ILSpy/App.axaml

@@ -118,6 +118,18 @@
             <Setter Property="Foreground" Value="White" />
         </Style>
 
+        <!-- Menu separators between MenuCategory groups. Simple theme's default is a thin
+             line at very low contrast against the menu background — barely visible. Boost
+             height and use a more contrasting brush so the MEF-injected groups (Save vs
+             Remove vs Exit in File menu, View vs Navigation vs Options in View menu, etc.)
+             read as distinct sections. Scoped to MenuItem-descendant separators so it
+             doesn't catch toolbar separators or the docking-chrome ones. -->
+        <Style Selector="MenuItem Separator">
+            <Setter Property="Height" Value="1" />
+            <Setter Property="Margin" Value="0,4" />
+            <Setter Property="Background" Value="Gray" />
+        </Style>
+
         <!-- VS-style preview tab: italicise the title of any DocumentTabStripItem whose
              dockable is a ContentTabPage with IsPreview=true. Tool-pane tabs and pinned
              document tabs fall through to Normal via the converter's null/false branch.