Mark NativeMenuItem.Menu as content property.

Means that submenus don't need the extra `<NativeMenuItem.Menu>` element.

samples/ControlCatalog/DecoratedWindow.xaml

@@ -6,25 +6,21 @@
         <NativeMenu.Menu>
     <NativeMenu>
       <NativeMenuItem Header="Decorated">
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-            <NativeMenuItem Header="Open"/>
-            <NativeMenuItem Header="Recent">
-              <NativeMenuItem.Menu>
-                <NativeMenu/>
-              </NativeMenuItem.Menu>
-            </NativeMenuItem>
-            <NativeMenuItem Header="Quit Avalonia" Gesture="CMD+Q"/>
-          </NativeMenu>
-        </NativeMenuItem.Menu>
+        <NativeMenu>
+          <NativeMenuItem Header="Open"/>
+          <NativeMenuItem Header="Recent">
+            <NativeMenuItem.Menu>
+              <NativeMenu/>
+            </NativeMenuItem.Menu>
+          </NativeMenuItem>
+          <NativeMenuItem Header="Quit Avalonia" Gesture="CMD+Q"/>
+        </NativeMenu>
       </NativeMenuItem>
       <NativeMenuItem Header="Edit">
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-            <NativeMenuItem Header="Copy"/>
-            <NativeMenuItem Header="Paste"/>
-          </NativeMenu>
-        </NativeMenuItem.Menu>
+        <NativeMenu>
+          <NativeMenuItem Header="Copy"/>
+          <NativeMenuItem Header="Paste"/>
+        </NativeMenu>
       </NativeMenuItem>
     </NativeMenu>
   </NativeMenu.Menu>

samples/ControlCatalog/MainWindow.xaml

@@ -16,47 +16,39 @@
   <NativeMenu.Menu>
     <NativeMenu>
       <NativeMenuItem Header="File">
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-            <NativeMenuItem Icon="/Assets/test_icon.ico" Header="Open" Clicked="OnOpenClicked" Gesture="Ctrl+O"/>
-            <NativeMenuItemSeperator/>
-            <NativeMenuItem Icon="/Assets/github_icon.png" Header="Recent">
-              <NativeMenuItem.Menu>
-                <NativeMenu/>
-              </NativeMenuItem.Menu>
-            </NativeMenuItem>
-            <NativeMenuItemSeperator/>
-            <NativeMenuItem Header="{x:Static local:MainWindow.MenuQuitHeader}"
-                            Gesture="{x:Static local:MainWindow.MenuQuitGesture}"
-                            Clicked="OnCloseClicked" />
-          </NativeMenu>
-        </NativeMenuItem.Menu>
+        <NativeMenu>
+          <NativeMenuItem Icon="/Assets/test_icon.ico" Header="Open" Clicked="OnOpenClicked" Gesture="Ctrl+O"/>
+          <NativeMenuItemSeperator/>
+          <NativeMenuItem Icon="/Assets/github_icon.png" Header="Recent">
+            <NativeMenu/>
+          </NativeMenuItem>
+          <NativeMenuItemSeperator/>
+          <NativeMenuItem Header="{x:Static local:MainWindow.MenuQuitHeader}"
+                          Gesture="{x:Static local:MainWindow.MenuQuitGesture}"
+                          Clicked="OnCloseClicked" />
+        </NativeMenu>
       </NativeMenuItem>
       <NativeMenuItem Header="Edit">
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-            <NativeMenuItem Header="Copy"/>
-            <NativeMenuItem Header="Paste"/>
-          </NativeMenu>
-        </NativeMenuItem.Menu>
+        <NativeMenu>
+          <NativeMenuItem Header="Copy"/>
+          <NativeMenuItem Header="Paste"/>
+        </NativeMenu>
       </NativeMenuItem>
       <NativeMenuItem Header="Options">
-        <NativeMenuItem.Menu>
-          <NativeMenu>
-           <NativeMenuItem Header="Check Me (None)" 
-                            Command="{Binding ToggleMenuItemCheckedCommand}"
-                            ToggleType="None"
-                            IsChecked="{Binding IsMenuItemChecked}"  />
-            <NativeMenuItem Header="Check Me (CheckBox)" 
-                            Command="{Binding ToggleMenuItemCheckedCommand}"
-                            ToggleType="CheckBox"
-                            IsChecked="{Binding IsMenuItemChecked}"  />
-            <NativeMenuItem Header="Check Me (Radio)" 
-                            Command="{Binding ToggleMenuItemCheckedCommand}"
-                            ToggleType="Radio"
-                            IsChecked="{Binding IsMenuItemChecked}"  />
-          </NativeMenu>
-        </NativeMenuItem.Menu>
+        <NativeMenu>
+          <NativeMenuItem Header="Check Me (None)" 
+                          Command="{Binding ToggleMenuItemCheckedCommand}"
+                          ToggleType="None"
+                          IsChecked="{Binding IsMenuItemChecked}"  />
+          <NativeMenuItem Header="Check Me (CheckBox)" 
+                          Command="{Binding ToggleMenuItemCheckedCommand}"
+                          ToggleType="CheckBox"
+                          IsChecked="{Binding IsMenuItemChecked}"  />
+          <NativeMenuItem Header="Check Me (Radio)" 
+                          Command="{Binding ToggleMenuItemCheckedCommand}"
+                          ToggleType="Radio"
+                          IsChecked="{Binding IsMenuItemChecked}"  />
+        </NativeMenu>
       </NativeMenuItem>
     </NativeMenu>
   </NativeMenu.Menu>

src/Avalonia.Controls/NativeMenuItem.cs

@@ -2,6 +2,7 @@ using System;
 using System.Windows.Input;
 using Avalonia.Input;
 using Avalonia.Media.Imaging;
+using Avalonia.Metadata;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls
@@ -62,6 +63,7 @@ namespace Avalonia.Controls
         public static readonly DirectProperty<NativeMenuItem, NativeMenu> MenuProperty =
             AvaloniaProperty.RegisterDirect<NativeMenuItem, NativeMenu>(nameof(Menu), o => o.Menu, (o, v) => o.Menu = v);
 
+        [Content]
         public NativeMenu Menu
         {
             get => _menu;