Implement transparency level hint list on X11.

src/Avalonia.X11/TransparencyHelper.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Collections.Generic;
 using Avalonia.Controls;
 
+#nullable enable
+
 namespace Avalonia.X11
 {
     internal class TransparencyHelper :  IDisposable, X11Globals.IGlobalsSubscriber
@@ -9,10 +12,10 @@ namespace Avalonia.X11
         private readonly IntPtr _window;
         private readonly X11Globals _globals;
         private WindowTransparencyLevel _currentLevel;
-        private WindowTransparencyLevel _requestedLevel;
+        private IReadOnlyList<WindowTransparencyLevel>? _requestedLevels;
         private bool _blurAtomsAreSet;
         
-        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+        public Action<WindowTransparencyLevel>? TransparencyLevelChanged { get; set; }
         public WindowTransparencyLevel CurrentLevel => _currentLevel;
 
         public TransparencyHelper(X11Info x11, IntPtr window, X11Globals globals)
@@ -23,28 +26,62 @@ namespace Avalonia.X11
             _globals.AddSubscriber(this);
         }
 
-        public void SetTransparencyRequest(WindowTransparencyLevel level)
+        public void SetTransparencyRequest(IReadOnlyList<WindowTransparencyLevel> levels)
         {
-            _requestedLevel = level;
-            UpdateTransparency();
-        }
+            WindowTransparencyLevel? newLevel = null;
+            _requestedLevels = levels;
+
+            foreach (var level in levels)
+            {
+                if (!IsSupported(level))
+                    continue;
+
+                SetBlur(level == WindowTransparencyLevel.Blur);
+                newLevel = level;
+                break;
+            }
+
+            // If no matching transparency level was found, revert to Transparent or None depending
+            // on whether composition is enabled.
+            if (!newLevel.HasValue)
+            {
+                newLevel = _globals.IsCompositionEnabled ? 
+                    WindowTransparencyLevel.Transparent :
+                    WindowTransparencyLevel.None;
+                SetBlur(false);
+            }
 
-        private void UpdateTransparency()
-        {
-            var newLevel = UpdateAtomsAndGetTransparency();
             if (newLevel != _currentLevel)
             {
-                _currentLevel = newLevel;
-                TransparencyLevelChanged?.Invoke(newLevel);
+                _currentLevel = newLevel.Value;
+                TransparencyLevelChanged?.Invoke(newLevel.Value);
             }
         }
-        
-        private WindowTransparencyLevel UpdateAtomsAndGetTransparency()
+
+        private bool IsSupported(WindowTransparencyLevel level)
         {
-            var blur = _requestedLevel == WindowTransparencyLevel.Blur ||
-                _requestedLevel == WindowTransparencyLevel.AcrylicBlur ||
-                _requestedLevel == WindowTransparencyLevel.Mica;
+            // None is suppported when composition is disabled.
+            if (level == WindowTransparencyLevel.None)
+                return !_globals.IsCompositionEnabled;
+
+            // Transparent is suppported when composition is enabled.
+            if (level == WindowTransparencyLevel.Transparent)
+                return _globals.IsCompositionEnabled;
+
+            // Blur is supported when composition is enabled and KWin is used.
+            if (level == WindowTransparencyLevel.Blur)
+                return _globals.IsCompositionEnabled && _globals.WmName == "KWin";
+            
+            return false;
+        }
 
+        private void UpdateTransparency()
+        {
+            SetTransparencyRequest(_requestedLevels ?? Array.Empty<WindowTransparencyLevel>());
+        }
+
+        private void SetBlur(bool blur)
+        {
             if (blur)
             {
                 if (!_blurAtomsAreSet)
@@ -63,15 +100,7 @@ namespace Avalonia.X11
                     _blurAtomsAreSet = false;
                 }
             }
-
-            if (!_globals.IsCompositionEnabled)
-                return WindowTransparencyLevel.None;
-            if (blur && CanBlur)
-                return WindowTransparencyLevel.Blur;
-            return WindowTransparencyLevel.Transparent;
         }
-
-        private bool CanBlur => _globals.WmName == "KWin" && _globals.IsCompositionEnabled;
         
         public void Dispose()
         {

src/Avalonia.X11/X11Window.cs

@@ -196,7 +196,7 @@ namespace Avalonia.X11
             _rawEventGrouper = new RawEventGrouper(DispatchInput, platform.EventGrouperDispatchQueue);
             
             _transparencyHelper = new TransparencyHelper(_x11, _handle, platform.Globals);
-            _transparencyHelper.SetTransparencyRequest(WindowTransparencyLevel.None);
+            _transparencyHelper.SetTransparencyRequest(Array.Empty<WindowTransparencyLevel>());
 
             CreateIC();
 
@@ -1307,9 +1307,9 @@ namespace Avalonia.X11
 
         public IPopupPositioner? PopupPositioner { get; }
 
-        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevel)
+        public void SetTransparencyLevelHint(IReadOnlyList<WindowTransparencyLevel> transparencyLevels)
         {
-            ////_transparencyHelper?.SetTransparencyRequest(transparencyLevel);
+            _transparencyHelper?.SetTransparencyRequest(transparencyLevels);
         }
 
         public void SetWindowManagerAddShadowHint(bool enabled)