X11 transparency reporting and blur support

src/Avalonia.X11/TransparencyHelper.cs

@@ -0,0 +1,83 @@
+using System;
+using Avalonia.Controls;
+
+namespace Avalonia.X11
+{
+    class TransparencyHelper :  IDisposable, X11Globals.IGlobalsSubscriber
+    {
+        private readonly X11Info _x11;
+        private readonly IntPtr _window;
+        private readonly X11Globals _globals;
+        private WindowTransparencyLevel _currentLevel;
+        private WindowTransparencyLevel _requestedLevel;
+        private bool _isCompositing;
+        private bool _blurAtomsAreSet;
+        
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+        public WindowTransparencyLevel CurrentLevel => _currentLevel;
+
+        public TransparencyHelper(X11Info x11, IntPtr window, X11Globals globals)
+        {
+            _x11 = x11;
+            _window = window;
+            _globals = globals;
+            _globals.AddSubscriber(this);
+        }
+
+        public void SetTransparencyRequest(WindowTransparencyLevel level)
+        {
+            _requestedLevel = level;
+            UpdateTransparency();
+        }
+
+        private void UpdateTransparency()
+        {
+            var newLevel = UpdateAtomsAndGetTransparency();
+            if (newLevel != _currentLevel)
+            {
+                _currentLevel = newLevel;
+                TransparencyLevelChanged?.Invoke(newLevel);
+            }
+        }
+        
+        private WindowTransparencyLevel UpdateAtomsAndGetTransparency()
+        {
+            if (_requestedLevel >= WindowTransparencyLevel.Blur)
+            {
+                if (!_blurAtomsAreSet)
+                {
+                    IntPtr value = IntPtr.Zero;
+                    XLib.XChangeProperty(_x11.Display, _window, _x11.Atoms._KDE_NET_WM_BLUR_BEHIND_REGION,
+                        _x11.Atoms.XA_CARDINAL, 32, PropertyMode.Replace, ref value, 1);
+                    _blurAtomsAreSet = true;
+                }
+            }
+            else
+            {
+                if (_blurAtomsAreSet)
+                {
+                    XLib.XDeleteProperty(_x11.Display, _window, _x11.Atoms._KDE_NET_WM_BLUR_BEHIND_REGION);
+                    _blurAtomsAreSet = false;
+                }
+            }
+
+            if (!_globals.IsCompositionEnabled)
+                return WindowTransparencyLevel.None;
+            if (_requestedLevel >= WindowTransparencyLevel.Blur && CanBlur)
+                return WindowTransparencyLevel.Blur;
+            return WindowTransparencyLevel.Transparent;
+        }
+
+
+        private bool CanBlur => _globals.WmName == "KWin" && _globals.IsCompositionEnabled;
+        
+        public void Dispose()
+        {
+            _globals.RemoveSubscriber(this);
+        }
+
+        void X11Globals.IGlobalsSubscriber.WmChanged(string wmName) => UpdateTransparency();
+
+        void X11Globals.IGlobalsSubscriber.CompositionChanged(bool compositing) => UpdateTransparency();
+    }
+}

src/Avalonia.X11/X11Atoms.cs

@@ -185,6 +185,8 @@ namespace Avalonia.X11
         public readonly IntPtr UTF8_STRING;
         public readonly IntPtr UTF16_STRING;
         public readonly IntPtr ATOM_PAIR;
+        public readonly IntPtr MANAGER;
+        public readonly IntPtr _KDE_NET_WM_BLUR_BEHIND_REGION;
 
 
         public X11Atoms(IntPtr display)

src/Avalonia.X11/X11Globals.cs

@@ -0,0 +1,183 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using static Avalonia.X11.XLib;
+namespace Avalonia.X11
+{
+    unsafe class X11Globals
+    {
+        private readonly AvaloniaX11Platform _plat;
+        private readonly int _screenNumber;
+        private readonly X11Info _x11;
+        private readonly IntPtr _rootWindow;
+        private readonly IntPtr _compositingAtom;
+        private readonly List<IGlobalsSubscriber> _subscribers = new List<IGlobalsSubscriber>();
+        
+        private string _wmName;
+        private IntPtr _compositionAtomOwner;
+        private bool _isCompositionEnabled;
+
+        public X11Globals(AvaloniaX11Platform plat)
+        {
+            _plat = plat;
+            _x11 = plat.Info;
+            _screenNumber = XDefaultScreen(_x11.Display);
+            _rootWindow = XRootWindow(_x11.Display, _screenNumber);
+            plat.Windows[_rootWindow] = OnRootWindowEvent;
+            XSelectInput(_x11.Display, _rootWindow,
+                new IntPtr((int)(EventMask.StructureNotifyMask | EventMask.PropertyChangeMask)));
+            _compositingAtom = XInternAtom(_x11.Display, "_NET_WM_CM_S" + _screenNumber, false);
+            UpdateWmName();
+            UpdateCompositingAtomOwner();
+        }
+        
+        public string WmName
+        {
+            get => _wmName;
+            private set
+            {
+                if (_wmName != value)
+                {
+                    _wmName = value;
+                    // The collection might change during enumeration
+                    foreach (var s in _subscribers.ToList()) 
+                        s.WmChanged(value);
+                }
+            }
+        }
+
+        private IntPtr CompositionAtomOwner
+        {
+            get => _compositionAtomOwner;
+            set
+            {
+                if (_compositionAtomOwner != value)
+                {
+                    _compositionAtomOwner = value;
+                    IsCompositionEnabled = _compositionAtomOwner != IntPtr.Zero;
+                }
+            }
+        }
+
+        public bool IsCompositionEnabled
+        {
+            get => _isCompositionEnabled;
+            set
+            {
+                if (_isCompositionEnabled != value)
+                {
+                    _isCompositionEnabled = value;
+                    // The collection might change during enumeration
+                    foreach (var s in _subscribers.ToList()) 
+                        s.CompositionChanged(value);
+                }
+            }
+        }
+
+
+        IntPtr GetSupportingWmCheck(IntPtr window)
+        {
+            XGetWindowProperty(_x11.Display, _rootWindow, _x11.Atoms._NET_SUPPORTING_WM_CHECK,
+                IntPtr.Zero, new IntPtr(IntPtr.Size), false,
+                _x11.Atoms.XA_WINDOW, out IntPtr actualType, out int actualFormat, out IntPtr nitems,
+                out IntPtr bytesAfter, out IntPtr prop);
+            if (nitems.ToInt32() != 1)
+                return IntPtr.Zero;
+            try
+            {
+                if (actualType != _x11.Atoms.XA_WINDOW)
+                    return IntPtr.Zero;
+                return *(IntPtr*)prop.ToPointer();
+            }
+            finally
+            {
+                XFree(prop);
+            }
+        }
+
+        void UpdateCompositingAtomOwner()
+        {
+            // This procedure is described in https://tronche.com/gui/x/icccm/sec-2.html#s-2.8
+            
+            // Check the server-side selection owner
+            var newOwner = XGetSelectionOwner(_x11.Display, _compositingAtom);
+            while (CompositionAtomOwner != newOwner)
+            {
+                // We have a new owner, unsubscribe from the previous one first
+                if (CompositionAtomOwner != IntPtr.Zero)
+                {
+                    _plat.Windows.Remove(CompositionAtomOwner);
+                    XSelectInput(_x11.Display, CompositionAtomOwner, IntPtr.Zero);
+                }
+
+                // Set it as the current owner and select input
+                CompositionAtomOwner = newOwner;
+                if (CompositionAtomOwner != IntPtr.Zero)
+                {
+                    _plat.Windows[newOwner] = HandleCompositionAtomOwnerEvents;
+                    XSelectInput(_x11.Display, CompositionAtomOwner, new IntPtr((int)(EventMask.StructureNotifyMask)));
+                }
+                
+                // Check for the new owner again and repeat the procedure if it was changed between XGetSelectionOwner and XSelectInput call
+                newOwner = XGetSelectionOwner(_x11.Display, _compositingAtom);
+            }
+        }
+
+        private void HandleCompositionAtomOwnerEvents(XEvent ev)
+        {
+            if(ev.type == XEventName.DestroyNotify)
+                UpdateCompositingAtomOwner();
+        }
+        
+        void UpdateWmName() => WmName = GetWmName();
+
+        string GetWmName()
+        {
+            var wm = GetSupportingWmCheck(_rootWindow);
+            if (wm == IntPtr.Zero || wm != GetSupportingWmCheck(wm))
+                return null;
+            XGetWindowProperty(_x11.Display, wm, _x11.Atoms._NET_WM_NAME,
+                IntPtr.Zero, new IntPtr(0x7fffffff),
+                false, _x11.Atoms.UTF8_STRING, out var actualType, out var actualFormat,
+                out var nitems, out _, out var prop);
+            if (nitems == IntPtr.Zero)
+                return null;
+            try
+            {
+                if (actualFormat != 8)
+                    return null;
+                return Marshal.PtrToStringAnsi(prop, nitems.ToInt32());
+            }
+            finally
+            {
+                XFree(prop);
+            }
+        }
+        
+        private void OnRootWindowEvent(XEvent ev)
+        {
+            if (ev.type == XEventName.PropertyNotify)
+            {
+                if(ev.PropertyEvent.atom == _x11.Atoms._NET_SUPPORTING_WM_CHECK)
+                    UpdateWmName();
+            }
+
+            if (ev.type == XEventName.ClientMessage)
+            {
+                if(ev.ClientMessageEvent.message_type == _x11.Atoms.MANAGER
+                    && ev.ClientMessageEvent.ptr2 == _compositingAtom)
+                    UpdateCompositingAtomOwner();
+            }
+        }
+        
+        public interface IGlobalsSubscriber
+        {
+            void WmChanged(string wmName);
+            void CompositionChanged(bool compositing);
+        }
+
+        public void AddSubscriber(IGlobalsSubscriber subscriber) => _subscribers.Add(subscriber);
+        public void RemoveSubscriber(IGlobalsSubscriber subscriber) => _subscribers.Remove(subscriber);
+    }
+}

src/Avalonia.X11/X11Platform.cs

@@ -26,6 +26,7 @@ namespace Avalonia.X11
         public IX11Screens X11Screens { get; private set; }
         public IScreenImpl Screens { get; private set; }
         public X11PlatformOptions Options { get; private set; }
+        public X11Globals Globals { get; private set; }
         public void Initialize(X11PlatformOptions options)
         {
             Options = options;
@@ -36,6 +37,7 @@ namespace Avalonia.X11
                 throw new Exception("XOpenDisplay failed");
             XError.Init();
             Info = new X11Info(Display, DeferredDisplay);
+            Globals = new X11Globals(this);
             //TODO: log
             if (options.UseDBusMenu)
                 DBusHelper.TryInitialize();

src/Avalonia.X11/X11Window.cs

@@ -43,6 +43,7 @@ namespace Avalonia.X11
         private bool _wasMappedAtLeastOnce = false;
         private double? _scalingOverride;
         private bool _disabled;
+        private TransparencyHelper _transparencyHelper;
 
         public object SyncRoot { get; } = new object();
 
@@ -176,6 +177,9 @@ namespace Avalonia.X11
             UpdateSizeHints(null);
             _xic = XCreateIC(_x11.Xim, XNames.XNInputStyle, XIMProperties.XIMPreeditNothing | XIMProperties.XIMStatusNothing,
                 XNames.XNClientWindow, _handle, IntPtr.Zero);
+            _transparencyHelper = new TransparencyHelper(_x11, _handle, platform.Globals);
+            _transparencyHelper.SetTransparencyRequest(WindowTransparencyLevel.None);
+
             XFlush(_x11.Display);
             if(_popup)
                 PopupPositioner = new ManagedPopupPositioner(new ManagedPopupPositionerPopupImplHelper(popupParent, MoveResize));
@@ -301,7 +305,11 @@ namespace Avalonia.X11
         public Func<bool> Closing { get; set; }
         public Action<WindowState> WindowStateChanged { get; set; }
 
-        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+        public Action<WindowTransparencyLevel> TransparencyLevelChanged
+        {
+            get => _transparencyHelper.TransparencyLevelChanged;
+            set => _transparencyHelper.TransparencyLevelChanged = value;
+        }
 
         public Action Closed { get; set; }
         public Action<PixelPoint> PositionChanged { get; set; }
@@ -1087,8 +1095,9 @@ namespace Avalonia.X11
         public IPopupPositioner PopupPositioner { get; }
         public ITopLevelNativeMenuExporter NativeMenuExporter { get; }
 
-        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
+        public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) =>
+            _transparencyHelper.SetTransparencyRequest(transparencyLevel);
 
-        public WindowTransparencyLevel TransparencyLevel { get; private set; }
+        public WindowTransparencyLevel TransparencyLevel => _transparencyHelper.CurrentLevel;
     }
 }