Add stubs for transparency in other toplevel impls.

src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs

@@ -49,6 +49,9 @@ namespace Avalonia.Controls.Embedding.Offscreen
         public Action<Rect> Paint { get; set; }
         public Action<Size> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
+
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+
         public void SetInputRoot(IInputRoot inputRoot) => InputRoot = inputRoot;
 
         public virtual Point PointToClient(PixelPoint point) => point.ToPoint(1);
@@ -61,6 +64,9 @@ namespace Avalonia.Controls.Embedding.Offscreen
 
         public Action Closed { get; set; }
         public abstract IMouseDevice MouseDevice { get; }
+
+        public WindowTransparencyLevel TransparencyLevel { get => WindowTransparencyLevel.None; set { } }
+
         public IPopupImpl CreatePopup() => null;
     }
 }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -37,6 +37,8 @@ namespace Avalonia.DesignerSupport.Remote
         public WindowState WindowState { get; set; }
         public Action<WindowState> WindowStateChanged { get; set; }
 
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+
         public WindowStub(IWindowImpl parent = null)
         {
             if (parent != null)
@@ -141,6 +143,8 @@ namespace Avalonia.DesignerSupport.Remote
         public IPopupPositioner PopupPositioner { get; }
 
         public Action GotInputWhenDisabled { get; set; }
+
+        public WindowTransparencyLevel TransparencyLevel { get => WindowTransparencyLevel.None; set { } }
     }
 
     class ClipboardStub : IClipboard

src/Avalonia.Native/WindowImplBase.cs

@@ -369,6 +369,8 @@ namespace Avalonia.Native
 
         Action<double> ITopLevelImpl.ScalingChanged { get; set; }
 
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+
         public IScreenImpl Screen { get; private set; }
 
         // TODO
@@ -389,6 +391,8 @@ namespace Avalonia.Native
             _native.BeginDragAndDropOperation(effects, point, clipboard, callback, sourceHandle);
         }
 
+        public WindowTransparencyLevel TransparencyLevel { get => WindowTransparencyLevel.None; set { } }
+
         public IPlatformHandle Handle { get; private set; }
     }
 }

src/Avalonia.X11/X11Window.cs

@@ -300,6 +300,9 @@ namespace Avalonia.X11
         public Action Activated { get; set; }
         public Func<bool> Closing { get; set; }
         public Action<WindowState> WindowStateChanged { get; set; }
+
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+
         public Action Closed { get; set; }
         public Action<PixelPoint> PositionChanged { get; set; }
 
@@ -1083,5 +1086,6 @@ namespace Avalonia.X11
 
         public IPopupPositioner PopupPositioner { get; }
         public ITopLevelNativeMenuExporter NativeMenuExporter { get; }
+        public WindowTransparencyLevel TransparencyLevel { get; set; }
     }
 }

src/Linux/Avalonia.LinuxFramebuffer/FramebufferToplevelImpl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.LinuxFramebuffer.Input;
@@ -70,6 +71,9 @@ namespace Avalonia.LinuxFramebuffer
         public Action<Rect> Paint { get; set; }
         public Action<Size> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
+
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+
         public Action Closed { get; set; }
         public event Action LostFocus
         {
@@ -78,5 +82,7 @@ namespace Avalonia.LinuxFramebuffer
         }
 
         public Size ScaledSize => _outputBackend.PixelSize.ToSize(Scaling);
+
+        public WindowTransparencyLevel TransparencyLevel { get => WindowTransparencyLevel.None; set { } }
     }
 }