Make separator MenuItem non-focusable.

src/Avalonia.Controls/MenuItem.cs

@@ -99,13 +99,13 @@ namespace Avalonia.Controls
             SelectableMixin.Attach<MenuItem>(IsSelectedProperty);
             CommandProperty.Changed.Subscribe(CommandChanged);
             FocusableProperty.OverrideDefaultValue<MenuItem>(true);
+            HeaderProperty.Changed.AddClassHandler<MenuItem>(x => x.HeaderChanged);
             IconProperty.Changed.AddClassHandler<MenuItem>(x => x.IconChanged);
             IsSelectedProperty.Changed.AddClassHandler<MenuItem>(x => x.IsSelectedChanged);
             ItemsPanelProperty.OverrideDefaultValue<MenuItem>(DefaultPanel);
             ClickEvent.AddClassHandler<MenuItem>(x => x.OnClick);
             SubmenuOpenedEvent.AddClassHandler<MenuItem>(x => x.OnSubmenuOpened);
             IsSubMenuOpenProperty.Changed.AddClassHandler<MenuItem>(x => x.SubMenuOpenChanged);
-            PseudoClass<MenuItem, object>(HeaderProperty, x => x as string == "-", ":separator");
         }
 
         public MenuItem()
@@ -420,6 +420,24 @@ namespace Avalonia.Controls
             IsEnabled = Command == null || Command.CanExecute(CommandParameter);
         }
 
+        /// <summary>
+        /// Called when the <see cref="Header"/> property changes.
+        /// </summary>
+        /// <param name="e">The property change event.</param>
+        private void HeaderChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.NewValue is string newValue && newValue == "-")
+            {
+                PseudoClasses.Add(":separator");
+                Focusable = false;
+            }
+            else if (e.OldValue is string oldValue && oldValue == "-")
+            {
+                PseudoClasses.Remove(":separator");
+                Focusable = true;
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="Icon"/> property changes.
         /// </summary>