Implement transparency level hint list on win32.

Other backends have the relevant code commented-out still.

src/Android/Avalonia.Android/Platform/SkiaPlatform/TopLevelImpl.cs

@@ -299,91 +299,91 @@ namespace Avalonia.Android.Platform.SkiaPlatform
 
         public double Scaling => RenderScaling;
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
         {
-            if (TransparencyLevel != transparencyLevel)
-            {
-                bool isBelowR = Build.VERSION.SdkInt < BuildVersionCodes.R;
-                bool isAboveR = Build.VERSION.SdkInt > BuildVersionCodes.R;
-                if (_view.Context is AvaloniaMainActivity activity)
-                {
-                    if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur ||
-                        transparencyLevel == WindowTransparencyLevel.Mica ||
-                        transparencyLevel == WindowTransparencyLevel.None)
-                    {
-                        if (!isBelowR)
-                        {
-                            activity.SetTranslucent(false);
-                        }
-                        if (isAboveR)
-                        {
-                            activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
-
-                            var attr = activity.Window?.Attributes;
-                            if (attr != null)
-                            {
-                                attr.BlurBehindRadius = 0;
-
-                                activity.Window.Attributes = attr;
-                            }
-                        }
-                        activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.White));
-
-                        if (transparencyLevel != WindowTransparencyLevel.None)
-                        {
-                            return;
-                        }
-                    }
-
-                    if (transparencyLevel == WindowTransparencyLevel.Transparent)
-                    {
-                        if (!isBelowR)
-                        {
-                            activity.SetTranslucent(true);
-                        }
-                        if (isAboveR)
-                        {
-                            activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
-
-                            var attr = activity.Window?.Attributes;
-                            if (attr != null)
-                            {
-                                attr.BlurBehindRadius = 0;
-
-                                activity.Window.Attributes = attr;
-                            }
-                        }
-                        activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.Transparent));
-                    }
-
-                    if (transparencyLevel == WindowTransparencyLevel.Blur)
-                    {
-                        if (isAboveR)
-                        {
-                            activity.SetTranslucent(true);
-                            activity.Window?.AddFlags(WindowManagerFlags.BlurBehind);
-
-                            var attr = activity.Window?.Attributes;
-                            if (attr != null)
-                            {
-                                attr.BlurBehindRadius = 120;
-
-                                activity.Window.Attributes = attr;
-                            }
-                            activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.Transparent));
-                        }
-                        else
-                        {
-                            activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
-                            activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.White));
-
-                            return;
-                        }
-                    }
-
-                    TransparencyLevel = transparencyLevel;
-                }
-            }
+            ////if (TransparencyLevel != transparencyLevel)
+            ////{
+            ////    bool isBelowR = Build.VERSION.SdkInt < BuildVersionCodes.R;
+            ////    bool isAboveR = Build.VERSION.SdkInt > BuildVersionCodes.R;
+            ////    if (_view.Context is AvaloniaMainActivity activity)
+            ////    {
+            ////        if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur ||
+            ////            transparencyLevel == WindowTransparencyLevel.Mica ||
+            ////            transparencyLevel == WindowTransparencyLevel.None)
+            ////        {
+            ////            if (!isBelowR)
+            ////            {
+            ////                activity.SetTranslucent(false);
+            ////            }
+            ////            if (isAboveR)
+            ////            {
+            ////                activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
+
+            ////                var attr = activity.Window?.Attributes;
+            ////                if (attr != null)
+            ////                {
+            ////                    attr.BlurBehindRadius = 0;
+
+            ////                    activity.Window.Attributes = attr;
+            ////                }
+            ////            }
+            ////            activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.White));
+
+            ////            if (transparencyLevel != WindowTransparencyLevel.None)
+            ////            {
+            ////                return;
+            ////            }
+            ////        }
+
+            ////        if (transparencyLevel == WindowTransparencyLevel.Transparent)
+            ////        {
+            ////            if (!isBelowR)
+            ////            {
+            ////                activity.SetTranslucent(true);
+            ////            }
+            ////            if (isAboveR)
+            ////            {
+            ////                activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
+
+            ////                var attr = activity.Window?.Attributes;
+            ////                if (attr != null)
+            ////                {
+            ////                    attr.BlurBehindRadius = 0;
+
+            ////                    activity.Window.Attributes = attr;
+            ////                }
+            ////            }
+            ////            activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.Transparent));
+            ////        }
+
+            ////        if (transparencyLevel == WindowTransparencyLevel.Blur)
+            ////        {
+            ////            if (isAboveR)
+            ////            {
+            ////                activity.SetTranslucent(true);
+            ////                activity.Window?.AddFlags(WindowManagerFlags.BlurBehind);
+
+            ////                var attr = activity.Window?.Attributes;
+            ////                if (attr != null)
+            ////                {
+            ////                    attr.BlurBehindRadius = 120;
+
+            ////                    activity.Window.Attributes = attr;
+            ////                }
+            ////                activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.Transparent));
+            ////            }
+            ////            else
+            ////            {
+            ////                activity.Window?.ClearFlags(WindowManagerFlags.BlurBehind);
+            ////                activity.Window.SetBackgroundDrawable(new ColorDrawable(Color.White));
+
+            ////                return;
+            ////            }
+            ////        }
+
+            ////        TransparencyLevel = transparencyLevel;
+            ////    }
+            ////}
         }
         
         public virtual object TryGetFeature(Type featureType)

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

@@ -89,7 +89,7 @@ namespace Avalonia.Controls.Embedding.Offscreen
         public Action? LostFocus { get; set; }
         public abstract IMouseDevice MouseDevice { get; }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel) { }
 
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
 

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -117,7 +117,7 @@ namespace Avalonia.Platform
         /// <summary>
         /// Sets the <see cref="WindowTransparencyLevel"/> hint of the TopLevel.
         /// </summary>
-        void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel);
+        void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel);
 
         /// <summary>
         /// Gets the current <see cref="WindowTransparencyLevel"/> of the TopLevel.

src/Avalonia.Controls/TopLevel.cs

@@ -523,8 +523,8 @@ namespace Avalonia.Controls
             {
                 if (PlatformImpl != null)
                 {
-                    ////PlatformImpl.SetTransparencyLevelHint(
-                    ////    change.GetNewValue<IReadOnlyList<WindowTransparencyLevel>>() ?? Array.Empty<WindowTransparencyLevel>());
+                    PlatformImpl.SetTransparencyLevelHint(
+                        change.GetNewValue<IReadOnlyList<WindowTransparencyLevel>>() ?? Array.Empty<WindowTransparencyLevel>());
                 }
             }
             else if (change.Property == ActualThemeVariantProperty)

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -179,7 +179,7 @@ namespace Avalonia.DesignerSupport.Remote
 
         public Action GotInputWhenDisabled { get; set; }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel) { }
 
         public void SetWindowManagerAddShadowHint(bool enabled)
         {

src/Avalonia.Native/WindowImplBase.cs

@@ -485,27 +485,27 @@ namespace Avalonia.Native
             _native?.BeginDragAndDropOperation(effects, point, clipboard, callback, sourceHandle);
         }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) 
-        {
-            if (TransparencyLevel != transparencyLevel)
-            {
-                if (transparencyLevel == WindowTransparencyLevel.Blur ||
-                    transparencyLevel == WindowTransparencyLevel.AcrylicBlur ||
-                    transparencyLevel == WindowTransparencyLevel.Mica)
-                {
-                    transparencyLevel = WindowTransparencyLevel.AcrylicBlur;
-                }
-
-                TransparencyLevel = transparencyLevel;
-
-                _native.SetTransparencyMode(transparencyLevel == WindowTransparencyLevel.None
-                    ? AvnWindowTransparencyMode.Opaque 
-                    : transparencyLevel == WindowTransparencyLevel.Transparent 
-                        ? AvnWindowTransparencyMode.Transparent
-                        : AvnWindowTransparencyMode.Blur);
-
-                TransparencyLevelChanged?.Invoke(TransparencyLevel);
-            }
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel) 
+        {
+            ////if (TransparencyLevel != transparencyLevel)
+            ////{
+            ////    if (transparencyLevel == WindowTransparencyLevel.Blur ||
+            ////        transparencyLevel == WindowTransparencyLevel.AcrylicBlur ||
+            ////        transparencyLevel == WindowTransparencyLevel.Mica)
+            ////    {
+            ////        transparencyLevel = WindowTransparencyLevel.AcrylicBlur;
+            ////    }
+
+            ////    TransparencyLevel = transparencyLevel;
+
+            ////    _native.SetTransparencyMode(transparencyLevel == WindowTransparencyLevel.None
+            ////        ? AvnWindowTransparencyMode.Opaque 
+            ////        : transparencyLevel == WindowTransparencyLevel.Transparent 
+            ////            ? AvnWindowTransparencyMode.Transparent
+            ////            : AvnWindowTransparencyMode.Blur);
+
+            ////    TransparencyLevelChanged?.Invoke(TransparencyLevel);
+            ////}
         }
 
         public WindowTransparencyLevel TransparencyLevel { get; private set; } = WindowTransparencyLevel.None;

src/Avalonia.X11/X11Window.cs

@@ -1307,8 +1307,10 @@ namespace Avalonia.X11
 
         public IPopupPositioner? PopupPositioner { get; }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) =>
-            _transparencyHelper?.SetTransparencyRequest(transparencyLevel);
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
+        {
+            ////_transparencyHelper?.SetTransparencyRequest(transparencyLevel);
+        }
 
         public void SetWindowManagerAddShadowHint(bool enabled)
         {

src/Browser/Avalonia.Browser/BrowserTopLevelImpl.cs

@@ -223,13 +223,13 @@ namespace Avalonia.Browser
             return null;
         }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
         {
-            if (transparencyLevel == WindowTransparencyLevel.None
-                || transparencyLevel == WindowTransparencyLevel.Transparent)
-            {
-                TransparencyLevel = transparencyLevel;
-            }
+            ////if (transparencyLevel == WindowTransparencyLevel.None
+            ////    || transparencyLevel == WindowTransparencyLevel.Transparent)
+            ////{
+            ////    TransparencyLevel = transparencyLevel;
+            ////}
         }
 
         public Size ClientSize => _clientSize;

src/Headless/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -337,7 +337,7 @@ namespace Avalonia.Headless
             
         }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
         {
             
         }

src/Linux/Avalonia.LinuxFramebuffer/FramebufferToplevelImpl.cs

@@ -75,7 +75,7 @@ using Avalonia.Rendering.Composition;
 
         public Size ScaledSize => _outputBackend.PixelSize.ToSize(RenderScaling);
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel) { }
 
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
 

src/Windows/Avalonia.Win32.Interop/Wpf/WpfTopLevelImpl.cs

@@ -247,7 +247,7 @@ namespace Avalonia.Win32.Interop.Wpf
 
         public IPopupImpl CreatePopup() => null;
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel) { }
 
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
 

src/Windows/Avalonia.Win32/PlatformConstants.cs

@@ -2,12 +2,13 @@ using System;
 
 namespace Avalonia.Win32
 {
-    public static class PlatformConstants
+    internal static class PlatformConstants
     {
         public const string WindowHandleType = "HWND";
         public const string CursorHandleType = "HCURSOR";
 
-        internal static readonly Version Windows8 = new Version(6, 2);
-        internal static readonly Version Windows7 = new Version(6, 1);
+        public static readonly Version Windows10 = new Version(10, 0);
+        public static readonly Version Windows8 = new Version(6, 2);
+        public static readonly Version Windows7 = new Version(6, 1);
     }
 }

src/Windows/Avalonia.Win32/WinRT/Composition/WinUiCompositionShared.cs

@@ -12,6 +12,7 @@ internal class WinUiCompositionShared : IDisposable
     public ICompositionBrush? MicaBrush { get; }
     public object SyncRoot { get; } = new();
 
+    public static readonly Version MinAcrylicVersion = new(10, 0, 15063);
     public static readonly Version MinHostBackdropVersion = new(10, 0, 22000);
     
     public WinUiCompositionShared(ICompositor compositor)

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics.CodeAnalysis;
@@ -25,6 +25,7 @@ using Avalonia.Win32.WinRT.Composition;
 using Avalonia.Win32.WinRT;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
 using Avalonia.Input.Platform;
+using System.Diagnostics;
 
 namespace Avalonia.Win32
 {
@@ -346,82 +347,119 @@ namespace Avalonia.Win32
             return null;
         }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevels)
         {
-            TransparencyLevel = EnableBlur(transparencyLevel);
-        }
+            var windowsVersion = Win32Platform.WindowsVersion;
 
-        private WindowTransparencyLevel EnableBlur(WindowTransparencyLevel transparencyLevel)
-        {
-            if (Win32Platform.WindowsVersion.Major >= 6)
-            {
-                if (DwmIsCompositionEnabled(out var compositionEnabled) != 0 || !compositionEnabled)
-                {
-                    return WindowTransparencyLevel.None;
-                }
-                else if (Win32Platform.WindowsVersion.Major >= 10)
-                {
-                    return Win10EnableBlur(transparencyLevel);
-                }
-                else if (Win32Platform.WindowsVersion.Minor >= 2)
-                {
-                    return Win8xEnableBlur(transparencyLevel);
-                }
-                else
-                {
-                    return Win7EnableBlur(transparencyLevel);
-                }
-            }
-            else
+            foreach (var level in transparencyLevels)
             {
-                return WindowTransparencyLevel.None;
+                if (!IsSupported(level, windowsVersion))
+                    continue;
+
+                if (level == WindowTransparencyLevel.Transparent)
+                    SetTransparencyTransparent(windowsVersion);
+                else if (level == WindowTransparencyLevel.Blur)
+                    SetTransparencyBlur(windowsVersion);
+                else if (level == WindowTransparencyLevel.AcrylicBlur)
+                    SetTransparencyAcrylicBlur(windowsVersion);
+                else if (level == WindowTransparencyLevel.Mica)
+                    SetTransparencyMica(windowsVersion);
+
+                TransparencyLevel = level;
+                break;
             }
         }
 
-        private WindowTransparencyLevel Win7EnableBlur(WindowTransparencyLevel transparencyLevel)
+        private bool IsSupported(WindowTransparencyLevel level, Version windowsVersion)
         {
-            if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur)
-            {
-                transparencyLevel = WindowTransparencyLevel.Blur;
-            }
+            // Only None is suppported when composition is disabled.
+            if (!_isUsingComposition)
+                return level == WindowTransparencyLevel.None;
+
+            // When composition is enabled, None is not supported because the backing visual always
+            // has an alpha channel
+            if (level == WindowTransparencyLevel.None)
+                return false;
+
+            // Transparent only supported on Windows 8+.
+            if (level == WindowTransparencyLevel.Transparent)
+                return windowsVersion >= PlatformConstants.Windows8;
+
+            // Blur only supported on Windows 8 and lower.
+            if (level == WindowTransparencyLevel.Blur)
+                return windowsVersion < PlatformConstants.Windows10;
+
+            // Acrylic is supported on Windows >= 10.0.15063.
+            if (level == WindowTransparencyLevel.AcrylicBlur)
+                return windowsVersion >= WinUiCompositionShared.MinAcrylicVersion;
 
-            var blurInfo = new DWM_BLURBEHIND(false);
+            // Mica is supported on Windows >= 10.0.22000.
+            if (level == WindowTransparencyLevel.Mica)
+                return windowsVersion >= WinUiCompositionShared.MinHostBackdropVersion;
 
-            if (transparencyLevel == WindowTransparencyLevel.Blur)
+            return false;
+        }
+
+        private void SetTransparencyTransparent(Version windowsVersion)
+        {
+            // Transparent only supported with composition on Windows 8+.
+            if (!_isUsingComposition || windowsVersion < PlatformConstants.Windows8)
+                return;
+
+            if (windowsVersion < PlatformConstants.Windows10)
             {
-                blurInfo = new DWM_BLURBEHIND(true);
+                // Some of the AccentState Enum's values have different meanings on Windows 8.x than on
+                // Windows 10, hence using ACCENT_ENABLE_BLURBEHIND to disable blurbehind  ¯\_(ツ)_/¯.
+                // Hey, I'm just porting what was here before.
+                SetAccentState(AccentState.ACCENT_ENABLE_BLURBEHIND);
+                var blurInfo = new DWM_BLURBEHIND(false);
+                DwmEnableBlurBehindWindow(_hwnd, ref blurInfo);
             }
 
+            SetUseHostBackdropBrush(false);
+            _blurHost?.SetBlur(BlurEffect.None);
+        }
+
+        private void SetTransparencyBlur(Version windowsVersion)
+        {
+            // Blur only supported with composition on Windows 8 and lower.
+            if (!_isUsingComposition || windowsVersion >= PlatformConstants.Windows10)
+                return;
+
+            // Some of the AccentState Enum's values have different meanings on Windows 8.x than on
+            // Windows 10.
+            SetAccentState(AccentState.ACCENT_DISABLED);
+            var blurInfo = new DWM_BLURBEHIND(true);
             DwmEnableBlurBehindWindow(_hwnd, ref blurInfo);
+        }
 
-            if (transparencyLevel == WindowTransparencyLevel.Transparent)
-            {
-                return WindowTransparencyLevel.None;
-            }
-            else
-            {
-                return transparencyLevel;
-            }
+        private void SetTransparencyAcrylicBlur(Version windowsVersion)
+        {
+            // Acrylic blur only supported with composition on Windows >= 10.0.15063.
+            if (!_isUsingComposition || windowsVersion < WinUiCompositionShared.MinAcrylicVersion)
+                return;
+
+            SetUseHostBackdropBrush(true);
+            _blurHost?.SetBlur(BlurEffect.Acrylic);
         }
 
-        private WindowTransparencyLevel Win8xEnableBlur(WindowTransparencyLevel transparencyLevel)
+        private void SetTransparencyMica(Version windowsVersion)
         {
-            var accent = new AccentPolicy();
-            var accentStructSize = Marshal.SizeOf<AccentPolicy>();
+            // Mica only supported with composition on Windows >= 10.0.22000.
+            if (!_isUsingComposition || windowsVersion < WinUiCompositionShared.MinHostBackdropVersion)
+                return;
 
-            if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur)
-            {
-                transparencyLevel = WindowTransparencyLevel.Blur;
-            }
+            SetUseHostBackdropBrush(false);
+            _blurHost?.SetBlur(BlurEffect.Mica);
+        }
 
-            if (transparencyLevel == WindowTransparencyLevel.Transparent)
-            {
-                accent.AccentState = AccentState.ACCENT_ENABLE_BLURBEHIND;
-            }
-            else
-            {
-                accent.AccentState = AccentState.ACCENT_DISABLED;
-            }
+        private void SetAccentState(AccentState state)
+        {
+            var accent = new AccentPolicy();
+            var accentStructSize = Marshal.SizeOf(accent);
+
+            //Some of the AccentState Enum's values have different meanings on Windows 8.x than on Windows 10
+            accent.AccentState = state;
 
             var accentPtr = Marshal.AllocHGlobal(accentStructSize);
             Marshal.StructureToPtr(accent, accentPtr, false);
@@ -432,96 +470,18 @@ namespace Avalonia.Win32
             data.Data = accentPtr;
 
             SetWindowCompositionAttribute(_hwnd, ref data);
-
             Marshal.FreeHGlobal(accentPtr);
-
-            if (transparencyLevel == WindowTransparencyLevel.Blur ||
-                transparencyLevel == WindowTransparencyLevel.AcrylicBlur ||
-                transparencyLevel == WindowTransparencyLevel.Mica)
-            {
-                Win7EnableBlur(transparencyLevel);
-            }
-
-            return transparencyLevel;
         }
 
-        private WindowTransparencyLevel Win10EnableBlur(WindowTransparencyLevel transparencyLevel)
+        private void SetUseHostBackdropBrush(bool useHostBackdropBrush)
         {
-            if (_isUsingComposition)
-            {
-                BlurEffect effect;
-
-                if (transparencyLevel == WindowTransparencyLevel.Mica)
-                    effect = BlurEffect.Mica;
-                else if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur)
-                    effect = BlurEffect.Acrylic;
-                else
-                    effect = BlurEffect.None;
-
-                if (Win32Platform.WindowsVersion >= WinUiCompositionShared.MinHostBackdropVersion)
-                {
-                    unsafe
-                    {
-                        int pvUseBackdropBrush = effect == BlurEffect.Acrylic ? 1 : 0;
-                        DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_USE_HOSTBACKDROPBRUSH, &pvUseBackdropBrush, sizeof(int));
-                    }
-                }
-
-                if (Win32Platform.WindowsVersion < WinUiCompositionShared.MinHostBackdropVersion && effect == BlurEffect.Mica)
-                {
-                    effect = BlurEffect.Acrylic;
-                }
-
-                _blurHost?.SetBlur(effect);
+            if (Win32Platform.WindowsVersion < WinUiCompositionShared.MinHostBackdropVersion)
+                return;
 
-                return transparencyLevel;
-            }
-            else
+            unsafe
             {
-                bool canUseAcrylic = Win32Platform.WindowsVersion.Major > 10 || Win32Platform.WindowsVersion.Build >= 19628;
-
-                var accent = new AccentPolicy();
-                var accentStructSize = Marshal.SizeOf<AccentPolicy>();
-
-                if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur && !canUseAcrylic)
-                {
-                    transparencyLevel = WindowTransparencyLevel.Blur;
-                }
-
-                if (transparencyLevel == WindowTransparencyLevel.Transparent)
-                {
-                    accent.AccentState = AccentState.ACCENT_ENABLE_TRANSPARENTGRADIENT;
-                }
-                else if (transparencyLevel == WindowTransparencyLevel.Blur)
-                {
-                    accent.AccentState = AccentState.ACCENT_ENABLE_BLURBEHIND;
-                }
-                else if (transparencyLevel == WindowTransparencyLevel.AcrylicBlur || transparencyLevel == WindowTransparencyLevel.Mica)
-                {
-                    accent.AccentState = AccentState.ACCENT_ENABLE_ACRYLIC;
-                    transparencyLevel = WindowTransparencyLevel.AcrylicBlur;
-                }
-                else
-                {
-                    accent.AccentState = AccentState.ACCENT_DISABLED;
-                }
-
-                accent.AccentFlags = 2;
-                accent.GradientColor = 0x01000000;
-
-                var accentPtr = Marshal.AllocHGlobal(accentStructSize);
-                Marshal.StructureToPtr(accent, accentPtr, false);
-
-                var data = new WindowCompositionAttributeData();
-                data.Attribute = WindowCompositionAttribute.WCA_ACCENT_POLICY;
-                data.SizeOfData = accentStructSize;
-                data.Data = accentPtr;
-
-                SetWindowCompositionAttribute(_hwnd, ref data);
-
-                Marshal.FreeHGlobal(accentPtr);
-
-                return transparencyLevel;
+                var pvUseBackdropBrush = useHostBackdropBrush ? 1 : 0;
+                DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_USE_HOSTBACKDROPBRUSH, &pvUseBackdropBrush, sizeof(int));
             }
         }
 

src/iOS/Avalonia.iOS/AvaloniaView.cs

@@ -139,7 +139,7 @@ namespace Avalonia.iOS
                 return null;
             }
 
-            public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+            public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
             {
                 // No-op
             }

tests/Avalonia.UnitTests/CompositorTestServices.cs

@@ -192,7 +192,7 @@ public class CompositorTestServices : IDisposable
         public IMouseDevice MouseDevice { get; } = new MouseDevice();
         public IPopupImpl CreatePopup() => throw new NotImplementedException();
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
         {
         }