Allow MenuItems to be separators.

If `MenuItem.Header == "-"` then apply the `:separator` pseudoclass to the `MenuItem` and display it as a separator.

samples/ControlCatalog/Pages/MenuPage.xaml.cs

@@ -14,16 +14,26 @@ namespace ControlCatalog.Pages
                 new MenuItemViewModel
                 {
                     Header = "_File",
-                    Items =
+                    Items = new[]
                     {
                         new MenuItemViewModel { Header = "_Open..." },
                         new MenuItemViewModel { Header = "Save" },
+                        new MenuItemViewModel { Header = "-" },
+                        new MenuItemViewModel
+                        {
+                            Header = "Recent",
+                            Items = new[]
+                            {
+                                new MenuItemViewModel { Header = "File1.txt" },
+                                new MenuItemViewModel { Header = "File2.txt" },
+                            }
+                        },
                     }
                 },
                 new MenuItemViewModel
                 {
                     Header = "_Edit",
-                    Items =
+                    Items = new[]
                     {
                         new MenuItemViewModel { Header = "_Copy" },
                         new MenuItemViewModel { Header = "_Paste" },
@@ -41,6 +51,6 @@ namespace ControlCatalog.Pages
     public class MenuItemViewModel
     {
         public string Header { get; set; }
-        public IList<MenuItemViewModel> Items { get; } = new List<MenuItemViewModel>();
+        public IList<MenuItemViewModel> Items { get; set; }
     }
 }

src/Avalonia.Controls/MenuItem.cs

@@ -105,6 +105,7 @@ namespace Avalonia.Controls
             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()
@@ -357,10 +358,21 @@ namespace Avalonia.Controls
         {
             base.OnTemplateApplied(e);
 
-            _popup = e.NameScope.Get<Popup>("PART_Popup");
-            _popup.DependencyResolver = DependencyResolver.Instance;
-            _popup.Opened += PopupOpened;
-            _popup.Closed += PopupClosed;
+            if (_popup != null)
+            {
+                _popup.Opened -= PopupOpened;
+                _popup.Closed -= PopupClosed;
+                _popup.DependencyResolver = null;
+            }
+
+            _popup = e.NameScope.Find<Popup>("PART_Popup");
+
+            if (_popup != null)
+            {
+                _popup.DependencyResolver = DependencyResolver.Instance;
+                _popup.Opened += PopupOpened;
+                _popup.Closed += PopupClosed;
+            }
         }
 
         /// <summary>

src/Avalonia.Themes.Default/MenuItem.xaml

@@ -73,7 +73,17 @@
       </ControlTemplate>
     </Setter>
   </Style>
-  
+
+  <Style Selector="MenuItem:separator">
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Separator Background="{DynamicResource ThemeControlMidBrush}"
+                   Margin="29,1,0,1"
+                   Height="1"/>
+      </ControlTemplate>
+    </Setter>
+  </Style>
+
   <Style Selector="Menu > MenuItem">
     <Setter Property="Template">
       <ControlTemplate>