indicate if managed decorations are needed via platform.

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -88,6 +88,8 @@ namespace Avalonia.DesignerSupport.Remote
 
         public Thickness OffScreenMargin { get; } = new Thickness();
 
+        public bool NeedsManagedDecorations => false;
+
         public void Activate()
         {
         }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -172,6 +172,8 @@ namespace Avalonia.DesignerSupport.Remote
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
 
         public bool IsClientAreaExtendedToDecorations { get; }
+
+        public bool NeedsManagedDecorations => false;
     }
 
     class ClipboardStub : IClipboard

src/Avalonia.Native/WindowImpl.cs

@@ -181,6 +181,8 @@ namespace Avalonia.Native
             ExtendClientAreaToDecorationsChanged?.Invoke(_isExtended);
         }
 
+        public bool NeedsManagedDecorations => false;
+
         public void ShowTaskbarIcon(bool value)
         {
             // NO OP On OSX

src/Avalonia.X11/X11Window.cs

@@ -1127,5 +1127,7 @@ namespace Avalonia.X11
         }
 
         public WindowTransparencyLevel TransparencyLevel => _transparencyHelper.CurrentLevel;
+
+        public bool NeedsManagedDecorations => false;
     }
 }