dont use property for setting transparency level hint.

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

@@ -65,7 +65,9 @@ namespace Avalonia.Controls.Embedding.Offscreen
         public Action Closed { get; set; }
         public abstract IMouseDevice MouseDevice { get; }
 
-        public WindowTransparencyLevel TransparencyLevelHint { get => WindowTransparencyLevel.None; set { } }
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
 
         public IPopupImpl CreatePopup() => null;
     }

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -114,8 +114,13 @@ namespace Avalonia.Platform
         IPopupImpl CreatePopup();
 
         /// <summary>
-        /// Gets or sets the <see cref="WindowTransparencyLevel"/> of the TopLevel.
+        /// Sets the <see cref="WindowTransparencyLevel"/> hint of the TopLevel.
         /// </summary>
-        WindowTransparencyLevel TransparencyLevelHint { get; set; }
+        void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel);
+
+        /// <summary>
+        /// Gets the current <see cref="WindowTransparencyLevel"/> of the TopLevel.
+        /// </summary>
+        WindowTransparencyLevel TransparencyLevel { get; }
     }
 }

src/Avalonia.Controls/TopLevel.cs

@@ -83,7 +83,7 @@ namespace Avalonia.Controls
             AffectsMeasure<TopLevel>(ClientSizeProperty);
 
             TransparencyLevelHintProperty.Changed.AddClassHandler<TopLevel>(
-                (tl, e) => { if (tl.PlatformImpl != null) tl.PlatformImpl.TransparencyLevelHint = (WindowTransparencyLevel)e.NewValue; });
+                (tl, e) => { if (tl.PlatformImpl != null) tl.PlatformImpl.SetTransparencyLevelHint((WindowTransparencyLevel)e.NewValue); });
         }
 
         /// <summary>
@@ -402,7 +402,7 @@ namespace Avalonia.Controls
 
             _transparencyFallbackBorder = e.NameScope.Find<Border>("PART_TransparencyFallback");
 
-            HandleTransparencyLevelChanged(PlatformImpl.TransparencyLevelHint);
+            HandleTransparencyLevelChanged(PlatformImpl.TransparencyLevel);
         }
 
         /// <summary>

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -144,7 +144,9 @@ namespace Avalonia.DesignerSupport.Remote
 
         public Action GotInputWhenDisabled { get; set; }
 
-        public WindowTransparencyLevel TransparencyLevelHint { get => WindowTransparencyLevel.None; set { } }
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
     }
 
     class ClipboardStub : IClipboard

src/Avalonia.Native/WindowImplBase.cs

@@ -391,23 +391,19 @@ namespace Avalonia.Native
             _native.BeginDragAndDropOperation(effects, point, clipboard, callback, sourceHandle);
         }
 
-        private WindowTransparencyLevel _transparencyLevel;
-
-        public WindowTransparencyLevel TransparencyLevelHint 
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) 
         {
-            get => _transparencyLevel;
-            set
+            if (TransparencyLevel != transparencyLevel)
             {
-                if(_transparencyLevel != value)
-                {
-                    _transparencyLevel = value;
+                TransparencyLevel = transparencyLevel;
 
-                    _native.SetBlurEnabled(_transparencyLevel >= WindowTransparencyLevel.Blur);
-                    TransparencyLevelChanged?.Invoke(_transparencyLevel);
-                }
-            } 
+                _native.SetBlurEnabled(TransparencyLevel >= WindowTransparencyLevel.Blur);
+                TransparencyLevelChanged?.Invoke(TransparencyLevel);
+            }
         }
 
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
+
         public IPlatformHandle Handle { get; private set; }
     }
 }

src/Avalonia.X11/X11Window.cs

@@ -1086,6 +1086,9 @@ namespace Avalonia.X11
 
         public IPopupPositioner PopupPositioner { get; }
         public ITopLevelNativeMenuExporter NativeMenuExporter { get; }
-        public WindowTransparencyLevel TransparencyLevelHint { get; set; }
+
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
     }
 }

src/Linux/Avalonia.LinuxFramebuffer/FramebufferToplevelImpl.cs

@@ -83,6 +83,8 @@ namespace Avalonia.LinuxFramebuffer
 
         public Size ScaledSize => _outputBackend.PixelSize.ToSize(Scaling);
 
-        public WindowTransparencyLevel TransparencyLevelHint { get => WindowTransparencyLevel.None; set { } }
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
     }
 }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -66,8 +66,7 @@ namespace Avalonia.Win32
         private OleDropTarget _dropTarget;
         private Size _minSize;
         private Size _maxSize;
-        private WindowImpl _parent;
-        private WindowTransparencyLevel _transparencyLevel = WindowTransparencyLevel.None;
+        private WindowImpl _parent;        
 
         public WindowImpl()
         {
@@ -210,18 +209,16 @@ namespace Avalonia.Win32
             }
         }
 
-        public WindowTransparencyLevel TransparencyLevelHint
+        public WindowTransparencyLevel TransparencyLevel { get; private set; }
+
+        public void SetTransparencyLevelHint (WindowTransparencyLevel transparencyLevel)
         {
-            get => _transparencyLevel;
-            set
-            {
-                var oldValue = _transparencyLevel;
-                _transparencyLevel = EnableBlur(value);
+            var level = EnableBlur(transparencyLevel);
 
-                if (oldValue != _transparencyLevel)
-                {
-                    TransparencyLevelChanged?.Invoke(_transparencyLevel);
-                }
+            if (TransparencyLevel != level)
+            {
+                TransparencyLevel = level;
+                TransparencyLevelChanged?.Invoke(TransparencyLevel);
             }
         }
 
@@ -281,7 +278,7 @@ namespace Avalonia.Win32
 
             SetWindowCompositionAttribute(_hwnd, ref data);
 
-            Marshal.FreeHGlobal(accentPtr);
+            Marshal.FreeHGlobal(accentPtr);            
 
             return transparencyLevel;
         }