Allow for setting wm shadow hint on a context menu.

src/Avalonia.Controls/ContextMenu.cs

@@ -62,6 +62,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<Rect?> PlacementRectProperty =
             AvaloniaProperty.Register<Popup, Rect?>(nameof(PlacementRect));
 
+        /// <summary>
+        /// Defines the <see cref="WindowManagerAddShadowHint"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> WindowManagerAddShadowHintProperty  =
+            Popup.WindowManagerAddShadowHintProperty.AddOwner<ContextMenu>();
+
         /// <summary>
         /// Defines the <see cref="PlacementTarget"/> property.
         /// </summary>
@@ -158,6 +164,12 @@ namespace Avalonia.Controls
             set { SetValue(PlacementModeProperty, value); }
         }
 
+        public bool WindowManagerAddShadowHint
+        {
+            get { return GetValue(WindowManagerAddShadowHintProperty); }
+            set { SetValue(WindowManagerAddShadowHintProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the the anchor rectangle within the parent that the context menu will be placed
         /// relative to when <see cref="PlacementMode"/> is <see cref="PlacementMode.AnchorAndGravity"/>.
@@ -267,6 +279,7 @@ namespace Avalonia.Controls
                     PlacementTarget = PlacementTarget ?? control,
                     IsLightDismissEnabled = true,
                     OverlayDismissEventPassThrough = true,
+                    WindowManagerAddShadowHint = WindowManagerAddShadowHint,
                 };
 
                 _popup.Opened += PopupOpened;