move popup hint property.

src/Avalonia.Controls/Primitives/Popup.cs

@@ -19,6 +19,9 @@ namespace Avalonia.Controls.Primitives
     /// </summary>
     public class Popup : Control, IVisualTreeHost
     {
+        public static readonly StyledProperty<bool> WindowManagerAddShadowHintProperty =
+            AvaloniaProperty.Register<PopupRoot, bool>(nameof(WindowManagerAddShadowHint), true);
+
         /// <summary>
         /// Defines the <see cref="Child"/> property.
         /// </summary>
@@ -89,7 +92,7 @@ namespace Avalonia.Controls.Primitives
         {
             IsHitTestVisibleProperty.OverrideDefaultValue<Popup>(false);
             ChildProperty.Changed.AddClassHandler<Popup>((x, e) => x.ChildChanged(e));
-            IsOpenProperty.Changed.AddClassHandler<Popup>((x, e) => x.IsOpenChanged((AvaloniaPropertyChangedEventArgs<bool>)e));
+            IsOpenProperty.Changed.AddClassHandler<Popup>((x, e) => x.IsOpenChanged((AvaloniaPropertyChangedEventArgs<bool>)e));            
         }
 
         /// <summary>
@@ -104,6 +107,12 @@ namespace Avalonia.Controls.Primitives
 
         public IPopupHost? Host => _openState?.PopupHost;
 
+        public bool WindowManagerAddShadowHint
+        {
+            get { return GetValue(WindowManagerAddShadowHintProperty); }
+            set { SetValue(WindowManagerAddShadowHintProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the control to display in the popup.
         /// </summary>
@@ -293,6 +302,8 @@ namespace Avalonia.Controls.Primitives
 
             _openState = new PopupOpenState(topLevel, popupHost, cleanupPopup);
 
+            WindowManagerAddShadowHintChanged(popupHost, WindowManagerAddShadowHint);
+
             popupHost.Show();
 
             using (BeginIgnoringIsOpen())
@@ -332,6 +343,14 @@ namespace Avalonia.Controls.Primitives
             return Disposable.Create((unsubscribe, target, handler), state => state.unsubscribe(state.target, state.handler));
         }
 
+        private void WindowManagerAddShadowHintChanged(IPopupHost host, bool hint)
+        {
+            if(host is PopupRoot pr)
+            {
+                pr.PlatformImpl.SetWindowManagerAddShadowHint(hint);
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="IsOpen"/> property changes.
         /// </summary>

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -17,19 +17,14 @@ namespace Avalonia.Controls.Primitives
     public sealed class PopupRoot : WindowBase, IInteractive, IHostedVisualTreeRoot, IDisposable, IStyleHost, IPopupHost
     {
         private readonly TopLevel _parent;
-        private PopupPositionerParameters _positionerParameters;
-
-        public static readonly StyledProperty<bool> WindowManagerAddShadowHintProperty =
-            AvaloniaProperty.Register<PopupRoot, bool>(nameof(WindowManagerAddShadowHint), true);
+        private PopupPositionerParameters _positionerParameters;        
 
         /// <summary>
         /// Initializes static members of the <see cref="PopupRoot"/> class.
         /// </summary>
         static PopupRoot()
         {
-            BackgroundProperty.OverrideDefaultValue(typeof(PopupRoot), Brushes.White);
-
-            WindowManagerAddShadowHintProperty.Changed.AddClassHandler<PopupRoot>((tl, e) => tl.PlatformImpl.SetWindowManagerAddShadowHint((bool)e.NewValue));
+            BackgroundProperty.OverrideDefaultValue(typeof(PopupRoot), Brushes.White);            
         }
 
         /// <summary>
@@ -58,13 +53,7 @@ namespace Avalonia.Controls.Primitives
         /// Gets the platform-specific window implementation.
         /// </summary>
         [CanBeNull]
-        public new IPopupImpl PlatformImpl => (IPopupImpl)base.PlatformImpl;
-        
-        public bool WindowManagerAddShadowHint
-        {
-            get { return GetValue(WindowManagerAddShadowHintProperty); }
-            set { SetValue(WindowManagerAddShadowHintProperty, value); }
-        }
+        public new IPopupImpl PlatformImpl => (IPopupImpl)base.PlatformImpl;               
 
         /// <summary>
         /// Gets the parent control in the event route.