Merge pull request #4448 from AvaloniaUI/fixes/menu-grid-sharedsizescope

Fix shared size group in Fluent sub-menu

src/Avalonia.Controls/MenuItem.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reactive.Linq;
 using System.Windows.Input;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Mixins;
@@ -120,6 +121,32 @@ namespace Avalonia.Controls
 
         public MenuItem()
         {
+            // HACK: This nasty but it's all WPF's fault. Grid uses an inherited attached
+            // property to store SharedSizeGroup state, except property inheritance is done
+            // down the logical tree. In this case, the control which is setting
+            // Grid.IsSharedSizeScope="True" is not in the logical tree. Instead of fixing
+            // the way Grid stores shared size state, the developers of WPF just created a
+            // binding of the internal state of the visual parent to the menu item. We don't
+            // have much choice but to do the same for now unless we want to refactor Grid,
+            // which I honestly am not brave enough to do right now. Here's the same hack in
+            // the WPF codebase:
+            //
+            // https://github.com/dotnet/wpf/blob/89537909bdf36bc918e88b37751add46a8980bb0/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/MenuItem.cs#L2126-L2141
+            //
+            // In addition to the hack from WPF, we also make sure to return null when we have
+            // no parent. If we don't do this, inheritance falls back to the logical tree,
+            // causing the shared size scope in the parent MenuItem to be used, breaking
+            // menu layout.
+
+            var parentSharedSizeScope = this.GetObservable(VisualParentProperty)
+                .SelectMany(x =>
+                {
+                    var parent = x as Control;
+                    return parent?.GetObservable(DefinitionBase.PrivateSharedSizeScopeProperty) ??
+                        Observable.Return<DefinitionBase.SharedSizeScope>(null);
+                });
+
+            this.Bind(DefinitionBase.PrivateSharedSizeScopeProperty, parentSharedSizeScope);
         }
 
         /// <summary>
@@ -326,27 +353,6 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            base.OnAttachedToVisualTree(e);
-
-            if (this.GetVisualParent() is IControl parent)
-            {
-                // HACK: This nasty but it's all WPF's fault. Grid uses an inherited attached
-                // property to store SharedSizeGroup state, except property inheritance is done
-                // down the logical tree. In this case, the control which is setting
-                // Grid.IsSharedSizeScope="True" is not in the logical tree. Instead of fixing
-                // the way Grid stores shared size state, the developers of WPF just created a
-                // binding of the internal state of the visual parent to the menu item. We don't
-                // have much choice but to do the same for now unless we want to refactor Grid,
-                // which I honestly am not brave enough to do right now. Here's the same hack in
-                // the WPF codebase:
-                //
-                // https://github.com/dotnet/wpf/blob/89537909bdf36bc918e88b37751add46a8980bb0/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/MenuItem.cs#L2126-L2141
-                SetValue(DefinitionBase.PrivateSharedSizeScopeProperty, parent.GetValue(DefinitionBase.PrivateSharedSizeScopeProperty));
-            }
-        }
-
         /// <summary>
         /// Called when the <see cref="MenuItem"/> is clicked.
         /// </summary>