fix acrylic backdrop on win11

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -765,6 +765,14 @@ namespace Avalonia.Win32.Interop
             DWMWA_CLOAK,
             DWMWA_CLOAKED,
             DWMWA_FREEZE_REPRESENTATION,
+            DWMWA_PASSIVE_UPDATE_MODE,
+            DWMWA_USE_HOSTBACKDROPBRUSH,
+            DWMWA_USE_IMMERSIVE_DARK_MODE = 20,
+            DWMWA_WINDOW_CORNER_PREFERENCE = 33,
+            DWMWA_BORDER_COLOR,
+            DWMWA_CAPTION_COLOR,
+            DWMWA_TEXT_COLOR,
+            DWMWA_VISIBLE_FRAME_BORDER_THICKNESS,
             DWMWA_LAST
         };
 
@@ -1456,6 +1464,9 @@ namespace Avalonia.Win32.Interop
         [DllImport("dwmapi.dll")]
         public static extern int DwmGetWindowAttribute(IntPtr hwnd, int dwAttribute, out RECT pvAttribute, int cbAttribute);
 
+        [DllImport("dwmapi.dll")]
+        public static extern int DwmSetWindowAttribute(IntPtr hwnd, int dwAttribute, void* pvAttribute, int cbAttribute);
+
         [DllImport("dwmapi.dll")]
         public static extern int DwmIsCompositionEnabled(out bool enabled);
 

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -379,13 +379,24 @@ namespace Avalonia.Win32
         {
             if (_isUsingComposition)
             {
-                _blurHost?.SetBlur(transparencyLevel switch
+                var effect = transparencyLevel switch
                 {
                     WindowTransparencyLevel.Mica => BlurEffect.Mica,
                     WindowTransparencyLevel.AcrylicBlur => BlurEffect.Acrylic,
                     WindowTransparencyLevel.Blur => BlurEffect.Acrylic,
                     _ => BlurEffect.None
-                });
+                };
+
+                if (Win32Platform.WindowsVersion >= WinUICompositorConnection.MinHostBackdropVersion)
+                {
+                    unsafe
+                    {
+                        int pvUseBackdropBrush = effect == BlurEffect.Acrylic ? 1 : 0;
+                        DwmSetWindowAttribute(_hwnd, (int)DwmWindowAttribute.DWMWA_USE_HOSTBACKDROPBRUSH, &pvUseBackdropBrush, sizeof(int));
+                    }
+                }
+
+                _blurHost?.SetBlur(effect);
 
                 return transparencyLevel;
             }