add hint for wm drop shadow.

src/Avalonia.Controls/Platform/IPopupImpl.cs

@@ -8,5 +8,7 @@ namespace Avalonia.Platform
     public interface IPopupImpl : IWindowBaseImpl
     {
         IPopupPositioner PopupPositioner { get; }
+
+        void SetWindowManagerAddShadowHint(bool enabled);
     }
 }

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -19,12 +19,17 @@ namespace Avalonia.Controls.Primitives
         private readonly TopLevel _parent;
         private PopupPositionerParameters _positionerParameters;
 
+        public static readonly StyledProperty<bool> WindowManagerAddShadowHintProperty =
+            AvaloniaProperty.Register<PopupRoot, bool>(nameof(WindowManagerAddShadowHint), true);
+
         /// <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));
         }
 
         /// <summary>
@@ -54,6 +59,12 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         [CanBeNull]
         public new IPopupImpl PlatformImpl => (IPopupImpl)base.PlatformImpl;
+        
+        public bool WindowManagerAddShadowHint
+        {
+            get { return GetValue(WindowManagerAddShadowHintProperty); }
+            set { SetValue(WindowManagerAddShadowHintProperty, value); }
+        }
 
         /// <summary>
         /// Gets the parent control in the event route.

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -146,6 +146,10 @@ namespace Avalonia.DesignerSupport.Remote
 
         public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
 
+        public void SetWindowManagerAddShadowHint(bool enabled)
+        {
+        }
+
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
     }
 

src/Avalonia.Native/PopupImpl.cs

@@ -59,6 +59,11 @@ namespace Avalonia.Native
         }
 
         public override IPopupImpl CreatePopup() => new PopupImpl(_factory, _opts, _glFeature, this);
+
+        public void SetWindowManagerAddShadowHint(bool enabled)
+        {
+        }
+
         public IPopupPositioner PopupPositioner { get; }
     }
 }

src/Avalonia.X11/X11Window.cs

@@ -1098,6 +1098,10 @@ namespace Avalonia.X11
         public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) =>
             _transparencyHelper.SetTransparencyRequest(transparencyLevel);
 
+        public void SetWindowManagerAddShadowHint(bool enabled)
+        {
+        }
+
         public WindowTransparencyLevel TransparencyLevel => _transparencyHelper.CurrentLevel;
     }
 }

src/Windows/Avalonia.Win32/PopupImpl.cs

@@ -7,6 +7,8 @@ namespace Avalonia.Win32
 {
     class PopupImpl : WindowImpl, IPopupImpl
     {
+        private bool _hasBoxShadow = true;
+
         public override void Show()
         {
             UnmanagedMethods.ShowWindow(Handle.Handle, UnmanagedMethods.ShowWindowCommand.ShowNoActivate);
@@ -36,11 +38,7 @@ namespace Avalonia.Win32
                 IntPtr.Zero,
                 IntPtr.Zero);
 
-            var classes = (int)UnmanagedMethods.GetClassLongPtr(result, (int)UnmanagedMethods.ClassLongIndex.GCL_STYLE);
-
-            classes |= (int)UnmanagedMethods.ClassStyles.CS_DROPSHADOW;
-
-            UnmanagedMethods.SetClassLong(result, UnmanagedMethods.ClassLongIndex.GCL_STYLE, new IntPtr(classes));
+            EnableBoxShadow(result, _hasBoxShadow);
 
             return result;
         }
@@ -68,6 +66,32 @@ namespace Avalonia.Win32
             //TODO: We ignore the scaling override for now
         }
 
+        private void EnableBoxShadow (IntPtr hwnd, bool enabled)
+        {
+            var classes = (int)UnmanagedMethods.GetClassLongPtr(hwnd, (int)UnmanagedMethods.ClassLongIndex.GCL_STYLE);
+
+            if (enabled)
+            {
+                classes |= (int)UnmanagedMethods.ClassStyles.CS_DROPSHADOW;
+            }
+            else
+            {
+                classes &= ~(int)UnmanagedMethods.ClassStyles.CS_DROPSHADOW;
+            }
+
+            UnmanagedMethods.SetClassLong(hwnd, UnmanagedMethods.ClassLongIndex.GCL_STYLE, new IntPtr(classes));
+        }
+
+        public void SetWindowManagerAddShadowHint(bool enabled)
+        {
+            _hasBoxShadow = enabled;
+
+            if (Handle != null)
+            {
+                EnableBoxShadow(Handle.Handle, enabled);
+            }
+        }
+
         public IPopupPositioner PopupPositioner { get; }
     }
 }