Add acrylic blur level with fallbacks to blur reported.

src/Avalonia.Controls/TopLevel.cs

@@ -376,13 +376,27 @@ namespace Avalonia.Controls
             LayoutHelper.InvalidateSelfAndChildrenMeasure(this);
         }
 
+        private bool TransparencyLevelsMatch (WindowTransparencyLevel requested, WindowTransparencyLevel received)
+        {
+            if(requested == received)
+            {
+                return true;
+            }
+            else if(requested >= WindowTransparencyLevel.Blur && received >= WindowTransparencyLevel.Blur)
+            {
+                return true;
+            }
+
+            return false;
+        }
+
         protected virtual void HandleTransparencyLevelChanged(WindowTransparencyLevel transparencyLevel)
         {
             if(_transparencyFallbackBorder != null)
             {
                 if(transparencyLevel == WindowTransparencyLevel.None || 
                     TransparencyLevelHint == WindowTransparencyLevel.None || 
-                    transparencyLevel != TransparencyLevelHint)
+                    !TransparencyLevelsMatch(TransparencyLevelHint, transparencyLevel))
                 {
                     _transparencyFallbackBorder.Background = TransparencyBackgroundFallback;
                 }

src/Avalonia.Controls/WindowTransparencyLevel.cs

@@ -15,6 +15,11 @@
         /// <summary>
         /// The window background is a blur-behind where nothing is drawn in the window.
         /// </summary>
-        Blur
+        Blur,
+
+        /// <summary>
+        /// The window background is a blur-behind with a high blur radius. This level may fallback to Blur.
+        /// </summary>
+        AcrylicBlur
     }
 }

src/Avalonia.Native/WindowImplBase.cs

@@ -395,6 +395,11 @@ namespace Avalonia.Native
         {
             if (TransparencyLevel != transparencyLevel)
             {
+                if (transparencyLevel >= WindowTransparencyLevel.Blur)
+                {
+                    transparencyLevel = WindowTransparencyLevel.AcrylicBlur;
+                }
+
                 TransparencyLevel = transparencyLevel;
 
                 _native.SetBlurEnabled(TransparencyLevel >= WindowTransparencyLevel.Blur);

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -239,6 +239,11 @@ namespace Avalonia.Win32
             var accent = new AccentPolicy();
             var accentStructSize = Marshal.SizeOf(accent);
 
+            if(transparencyLevel == WindowTransparencyLevel.AcrylicBlur && !canUseAcrylic)
+            {
+                transparencyLevel = WindowTransparencyLevel.Blur;
+            }
+
             switch (transparencyLevel)
             {
                 default:
@@ -251,11 +256,13 @@ namespace Avalonia.Win32
                     break;
 
                 case WindowTransparencyLevel.Blur:
-                    accent.AccentState = canUseAcrylic ? AccentState.ACCENT_ENABLE_ACRYLIC : AccentState.ACCENT_ENABLE_BLURBEHIND;
+                    accent.AccentState = AccentState.ACCENT_ENABLE_BLURBEHIND;
                     break;
 
-                case (WindowTransparencyLevel.Blur + 1): // hack force acrylic on windows 10.
+                case WindowTransparencyLevel.AcrylicBlur:
+                case (WindowTransparencyLevel.AcrylicBlur + 1): // hack-force acrylic.
                     accent.AccentState = AccentState.ACCENT_ENABLE_ACRYLIC;
+                    transparencyLevel = WindowTransparencyLevel.AcrylicBlur;
                     break;
             }