Let the native menu derive item enabled-state from CanExecute

The menu builder hard-set each NativeMenuItem's IsEnabled from static
export metadata (always true), overriding the state that assigning Command
would otherwise derive from the command's CanExecute. The inline
NativeMenuBar on Windows/Linux re-derives from the command and greys out
OS-gated items (e.g. Open from GAC, which is Windows-only), but the macOS
native menu reads NativeMenuItem.IsEnabled directly -- so those items
stayed enabled there even though invoking them is a no-op. Drop the
explicit IsEnabled so every command-backed item follows its CanExecute on
all platforms.

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

ILSpy.Tests/Views/MainMenuTests.cs

@@ -0,0 +1,70 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.Linq;
+
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+
+using AwesomeAssertions;
+
+using ILSpy;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.Views;
+
+/// <summary>
+/// The main menu is built as a NativeMenu (projected into the AppKit menu bar on macOS, rendered
+/// inline by NativeMenuBar on Windows/Linux). A menu item's enabled state must follow its
+/// command's CanExecute, not a hard-coded flag -- otherwise OS-gated commands like "Open from GAC"
+/// (Windows-only) stay clickable on the macOS native menu even though invoking them is a no-op.
+/// </summary>
+[TestFixture]
+public class MainMenuTests
+{
+	static NativeMenuItem? Find(NativeMenu menu, Func<NativeMenuItem, bool> predicate)
+	{
+		foreach (var item in menu.Items.OfType<NativeMenuItem>())
+		{
+			if (predicate(item))
+				return item;
+			if (item.Menu is { } submenu && Find(submenu, predicate) is { } hit)
+				return hit;
+		}
+		return null;
+	}
+
+	[AvaloniaTest]
+	public void OpenFromGac_item_enabled_state_follows_the_command()
+	{
+		var window = new Window();
+		MainMenu.Attach(window);
+
+		var menu = NativeMenu.GetMenu(window);
+		menu.Should().NotBeNull("the composition host is up in tests, so Attach builds the menu");
+
+		var gac = Find(menu!, i => i.Header?.Contains("GAC", StringComparison.OrdinalIgnoreCase) == true);
+		gac.Should().NotBeNull("the File menu must contain an 'Open from GAC' item");
+
+		gac!.IsEnabled.Should().Be(OperatingSystem.IsWindows(),
+			"the GAC is Windows-only, so the item must reflect the command's CanExecute (disabled off "
+			+ "Windows) instead of a hard-coded enabled flag that the macOS native menu would show wrongly");
+	}
+}

ILSpy/Views/MainMenu.axaml.cs

@@ -229,10 +229,14 @@ public static class MainMenu
 					else
 					{
 						var command = entry.CreateExport().Value;
+						// No explicit IsEnabled: assigning Command lets NativeMenuItem track the
+						// command's CanExecute, so OS-gated commands (e.g. Open from GAC, which is
+						// Windows-only) grey out correctly. A hard-coded IsEnabled would override
+						// that, and the macOS native menu reads it directly -- leaving the item
+						// wrongly enabled there even though invoking it is a no-op.
 						var menuItem = new NativeMenuItem {
 							Header = ResourceHelper.GetString(entry.Metadata?.Header),
 							Command = command,
-							IsEnabled = entry.Metadata?.IsEnabled ?? true,
 						};
 
 						// NativeMenuItem.Icon is Bitmap (macOS NSImage has no vector form),