fix: detach platform input when presentation source closes (#21782)

* test: cover platform input detachment on close

Closed top levels retain the presentation source as the platform input callback. A late platform input event can therefore target an already disposed visual tree.\n\nThe regression test requires top-level closure to clear ITopLevelImpl.Input.

* fix: detach platform input when presentation source closes

Win32 can deliver WM_CAPTURECHANGED while destroying a popup that owns mouse capture. The platform then calls the input handler retained by the disposed presentation source, which logs a warning and can route input into a detached visual tree.\n\nClear the platform input callback at the start of presentation source disposal, alongside the existing scaling event unsubscription.

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

Author: NathanDrake2406

src/Avalonia.Controls/PresentationSource/PresentationSource.cs

@@ -77,14 +77,19 @@ public InputElement RootVisual
 
     public void Dispose()
     {
+        if (PlatformImpl is { } platformImpl)
+        {
+            platformImpl.Input = null;
+            platformImpl.ScalingChanged -= HandleScalingChanged;
+        }
+
         _layoutDiagnosticBridge?.Dispose();
         _layoutDiagnosticBridge = null;
         LayoutManager.Dispose();
         Renderer.SceneInvalidated -= SceneInvalidated;
         // We need to wait for the renderer to complete any in-flight operations
         Renderer.Dispose();
 
-        PlatformImpl?.ScalingChanged -= HandleScalingChanged;
         PlatformImpl = null;
         _pointerOverPreProcessor?.OnCompleted();
         _pointerOverPreProcessorSubscription?.Dispose();

tests/Avalonia.Controls.UnitTests/PresentationSourceTests.cs

@@ -19,6 +19,20 @@ namespace Avalonia.Controls.UnitTests;
 
 public sealed class PresentationSourceTests : ScopedTestBase
 {
+    [Fact]
+    public void Closing_Should_Detach_Platform_Input_Handler()
+    {
+        using var app = UnitTestApplication.Start(TestServices.StyledWindow);
+        var windowImpl = MockWindowingPlatform.CreateWindowMock();
+        var window = new Window(windowImpl.Object);
+
+        Assert.NotNull(windowImpl.Object.Input);
+
+        windowImpl.Object.Closed!();
+
+        Assert.Null(windowImpl.Object.Input);
+    }
+
     [Fact]
     public void ChromeHitTest_Prefers_Overlay_Over_Content()
     {