Store X11Window instances inside X11Platform

src/Avalonia.X11/Dispatching/X11EventDispatcher.cs

@@ -8,16 +8,16 @@ internal class X11EventDispatcher
 {
     private readonly AvaloniaX11Platform _platform;
     private readonly IntPtr _display;
+    private readonly Dictionary<IntPtr, X11WindowInfo> _windows;
 
     public delegate void EventHandler(ref XEvent xev);
     public int Fd { get; }
-    private readonly Dictionary<IntPtr, EventHandler> _eventHandlers;
 
     public X11EventDispatcher(AvaloniaX11Platform platform)
     {
         _platform = platform;
         _display = platform.Display;
-        _eventHandlers = platform.Windows;
+        _windows = platform.Windows;
         Fd = XLib.XConnectionNumber(_display);
     }
 
@@ -46,8 +46,8 @@ internal class X11EventDispatcher
                         _platform.XI2.OnEvent((XIEvent*)xev.GenericEventCookie.data);
                     }
                 }
-                else if (_eventHandlers.TryGetValue(xev.AnyEvent.window, out var handler))
-                    handler(ref xev);
+                else if (_windows.TryGetValue(xev.AnyEvent.window, out var windowInfo))
+                    windowInfo.EventHandler(ref xev);
             }
             finally
             {
@@ -59,4 +59,4 @@ internal class X11EventDispatcher
     }
 
     public void Flush() => XFlush(_display);
-}
+}

src/Avalonia.X11/Selections/Clipboard/EventStreamWindow.cs

@@ -27,7 +27,7 @@ internal sealed class EventStreamWindow : IXEventWaiter
         {
             _isForeign = true;
             _handle = foreignWindow.Value;
-            _platform.Windows[_handle] = OnEvent;
+            _platform.Windows[_handle] = new X11WindowInfo(OnEvent, null);
         }
         else
             _handle = XLib.CreateEventWindow(platform, OnEvent);

src/Avalonia.X11/Selections/DragDrop/X11DragSource.cs

@@ -44,7 +44,7 @@ internal sealed class X11DragSource(AvaloniaX11Platform platform) : IPlatformDra
         private readonly TaskCompletionSource<DragDropEffects> _completionSource = new();
         private readonly DragDropDataProvider _dataProvider;
         private readonly IntPtr[] _formatAtoms;
-        private X11EventDispatcher.EventHandler? _originalEventHandler;
+        private X11WindowInfo? _originalSourceWindowInfo;
         private bool _pointerGrabbed;
         private XdndTargetInfo? _lastTarget;
         private IntPtr _lastStatusAction;
@@ -61,7 +61,7 @@ internal sealed class X11DragSource(AvaloniaX11Platform platform) : IPlatformDra
             _allowedEffects = allowedEffects;
             _dataProvider = new DragDropDataProvider(platform, dataTransfer);
 
-            if (!platform.Windows.TryGetValue(sourceWindow, out _originalEventHandler))
+            if (!platform.Windows.TryGetValue(sourceWindow, out var sourceWindowInfo))
             {
                 _formatAtoms = [];
                 _completionSource.TrySetResult(DragDropEffects.None);
@@ -93,7 +93,10 @@ internal sealed class X11DragSource(AvaloniaX11Platform platform) : IPlatformDra
             }
 
             _pointerGrabbed = true;
-            _platform.Windows[_sourceWindow] = OnEvent;
+
+            // Replace the window event handler with our own during the drag operation
+            _originalSourceWindowInfo = sourceWindowInfo;
+            _platform.Windows[_sourceWindow] = new X11WindowInfo(OnEvent, sourceWindowInfo.Window);
 
             var atoms = _platform.Info.Atoms;
             _formatAtoms = DataFormatHelper.ToAtoms(dataTransfer.Formats, atoms);
@@ -130,14 +133,14 @@ internal sealed class X11DragSource(AvaloniaX11Platform platform) : IPlatformDra
                 else if (message.message_type == atoms.XdndFinished)
                     OnXdndFinished(in message);
                 else
-                    _originalEventHandler?.Invoke(ref evt);
+                    _originalSourceWindowInfo?.EventHandler(ref evt);
             }
 
             else if (evt.type == XEventName.SelectionRequest)
                 _dataProvider.OnSelectionRequest(in evt.SelectionRequestEvent);
 
             else
-                _originalEventHandler?.Invoke(ref evt);
+                _originalSourceWindowInfo?.EventHandler(ref evt);
         }
 
         private void OnMotionNotify(in XMotionEvent motion)
@@ -337,10 +340,11 @@ internal sealed class X11DragSource(AvaloniaX11Platform platform) : IPlatformDra
             if (_pointerGrabbed)
                 UngrabPointer();
 
-            if (_originalEventHandler is not null && _platform.Windows.ContainsKey(_sourceWindow))
+            if (_originalSourceWindowInfo is { } originalSourceWindowInfo &&
+                _platform.Windows.ContainsKey(_sourceWindow))
             {
-                _platform.Windows[_sourceWindow] = _originalEventHandler;
-                _originalEventHandler = null;
+                _platform.Windows[_sourceWindow] = originalSourceWindowInfo;
+                _originalSourceWindowInfo = null;
             }
 
             _lastTarget = null;

src/Avalonia.X11/X11FocusProxy.cs

@@ -41,7 +41,7 @@ namespace Avalonia.X11
             _handle = PrepareXWindow(platform.Info.Display, parent);
             _platform = platform;
             _ownerEventHandler = eventHandler;
-            _platform.Windows[_handle] = OnEvent;
+            _platform.Windows[_handle] = new X11WindowInfo(OnEvent, null);
         }
 
         internal void Cleanup()

src/Avalonia.X11/X11Globals.cs

@@ -40,7 +40,7 @@ namespace Avalonia.X11
             _x11 = plat.Info;
             _screenNumber = XDefaultScreen(_x11.Display);
             _rootWindow = XRootWindow(_x11.Display, _screenNumber);
-            plat.Windows[_rootWindow] = OnRootWindowEvent;
+            plat.Windows[_rootWindow] = new X11WindowInfo(OnRootWindowEvent, null);
             XSelectInput(_x11.Display, _rootWindow,
                 new IntPtr((int)(EventMask.StructureNotifyMask | EventMask.PropertyChangeMask)));
             
@@ -156,7 +156,7 @@ namespace Avalonia.X11
                 CompositionAtomOwner = newOwner;
                 if (CompositionAtomOwner != IntPtr.Zero)
                 {
-                    _plat.Windows[newOwner] = HandleCompositionAtomOwnerEvents;
+                    _plat.Windows[newOwner] = new X11WindowInfo(HandleCompositionAtomOwnerEvents, null);
                     XSelectInput(_x11.Display, CompositionAtomOwner, new IntPtr((int)(EventMask.StructureNotifyMask)));
                 }
                 

src/Avalonia.X11/X11Platform.cs

@@ -33,7 +33,7 @@ namespace Avalonia.X11
         private X11AtSpiAccessibility? _accessibility;
         internal AtSpiServer? AtSpiServer => _accessibility?.Server;
         public KeyboardDevice KeyboardDevice => _keyboardDevice.Value;
-        public Dictionary<IntPtr, X11EventDispatcher.EventHandler> Windows { get; } = new ();
+        public Dictionary<IntPtr, X11WindowInfo> Windows { get; } = new ();
         public XI2Manager? XI2 { get; private set; }
         public X11Info Info { get; private set; } = null!;
         public X11Screens X11Screens { get; private set; } = null!;

src/Avalonia.X11/X11Window.cs

@@ -193,7 +193,7 @@ namespace Avalonia.X11
             _mode.OnHandleCreated(_handle);
             
             _realSize = new PixelSize(defaultWidth, defaultHeight);
-            platform.Windows[_handle] = OnEvent;
+            platform.Windows[_handle] = new X11WindowInfo(OnEvent, this);
             XEventMask ignoredMask = XEventMask.SubstructureRedirectMask
                                      | XEventMask.ResizeRedirectMask
                                      | XEventMask.PointerMotionHintMask;

src/Avalonia.X11/X11WindowInfo.cs

@@ -0,0 +1,7 @@
+namespace Avalonia.X11;
+
+internal readonly struct X11WindowInfo(X11EventDispatcher.EventHandler eventHandler, X11Window? window)
+{
+    public X11EventDispatcher.EventHandler EventHandler { get; } = eventHandler;
+    public X11Window? Window { get; } = window;
+}

src/Avalonia.X11/XLib.cs

@@ -751,7 +751,7 @@ namespace Avalonia.X11
         {
             var win = XCreateSimpleWindow(plat.Display, plat.Info.DefaultRootWindow, 
                 0, 0, 1, 1, 0, IntPtr.Zero, IntPtr.Zero);
-            plat.Windows[win] = handler;
+            plat.Windows[win] = new X11WindowInfo(handler, null);
             return win;
         }