Fix pointer capture cancellation on Win32 (#22054)

* Fix pointer capture cancellation on Win32

A cancelled pen or mouse interaction was reported as an ordinary leave,
and nothing released the capture that came with it, so the captured
element never saw PointerCaptureLost.

Win32 constants: POINTER_FLAG_CONFIDENCE and POINTER_FLAG_CANCELED were
declared an order of magnitude too small - the SDK defines them as
0x4000 and 0x8000 - so the cancellation check in GetEventType could
never match. Ctrl+Alt+Del or Win+L mid stroke therefore arrived as a
normal button up, and the app committed an interaction the user had
aborted.

Win32 handling: WM_POINTERCAPTURECHANGED and POINTER_FLAG_CANCELED now
map to CancelCapture for non touch pointers, matching what the legacy
WM_CAPTURECHANGED path has always done.

PenDevice: handle CancelCapture by releasing the capture while keeping
the pointer alive, since the pen is still in range and a leave arrives
separately.

Pointer hardening: CaptureLost ends every capture the pointer holds, on
the element and on a gesture recognizer, and the devices release through
it instead of repeating the steps. Capturing to a recognizer clears
Captured, so the null check in the platform path used to skip the whole
release while a scroll, pull or swipe gesture owned the pointer, and a
touch up never cleared the recognizer at all. Dispose releases as well
rather than leaving that to every caller, a disposed pointer ignores
further capture calls and asserts when a capturer is passed, and devices
drop a pointer from their map before disposing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Keep the explicit capture source in MouseTestHelper

The helper captures through Pointer.Capture(target), which is an
explicit capture, and used to release with Capture(null) - explicit as
well. Releasing as implicit changed the source that capture changing
handlers observe.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Avalonia.Base/Input/MouseDevice.cs

@@ -220,9 +220,7 @@ namespace Avalonia.Input
                 }
                 finally
                 {
-                    _pointer.Capture(null, CaptureSource.Implicit);
-                    _pointer.CaptureGestureRecognizer(null);
-                    _pointer.IsGestureRecognitionSkipped = false;
+                    _pointer.CaptureLost(CaptureSource.Implicit);
                     _lastMouseDownButton = default;
                 }
                 return e.Handled;

src/Avalonia.Base/Input/PenDevice.cs

@@ -63,6 +63,9 @@ namespace Avalonia.Input
                     case RawPointerEventType.LeaveWindow:
                         shouldReleasePointer = true;
                         break;
+                    case RawPointerEventType.CancelCapture:
+                        pointer.PlatformCaptureLost();
+                        break;
                     case RawPointerEventType.LeftButtonDown:
                     case RawPointerEventType.RightButtonDown:
                     case RawPointerEventType.MiddleButtonDown:
@@ -90,8 +93,8 @@ namespace Avalonia.Input
             {
                 if (shouldReleasePointer)
                 {
-                    pointer.Dispose();
                     _pointers.Remove(e.RawPointerId);
+                    pointer.Dispose();
                 }
             }
         }
@@ -173,9 +176,7 @@ namespace Avalonia.Input
                 }
                 finally
                 {
-                    pointer.Capture(null, CaptureSource.Implicit);
-                    pointer.CaptureGestureRecognizer(null);
-                    pointer.IsGestureRecognitionSkipped = false;
+                    pointer.CaptureLost(CaptureSource.Implicit);
                     _lastMouseDownButton = default;
                 }
 

src/Avalonia.Base/Input/Pointer.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using Avalonia.Input.GestureRecognizers;
 using Avalonia.VisualTree;
@@ -18,6 +19,8 @@ namespace Avalonia.Input
         private static int s_NextFreePointerId = 1000;
         public static int GetNextFreeId() => s_NextFreePointerId++;
         
+        private bool _disposed;
+
         public Pointer(int id, PointerType type, bool isPrimary)
         {
             Id = id;
@@ -40,10 +43,25 @@ namespace Avalonia.Input
 
         }
 
-        internal void PlatformCaptureLost()
+        internal void PlatformCaptureLost() => CaptureLost(CaptureSource.Platform);
+
+        /// <summary>
+        /// Ends every capture the pointer holds, on the element and on a gesture recognizer.
+        /// </summary>
+        internal void CaptureLost(CaptureSource source)
+        {
+            if (_disposed)
+                return;
+
+            CaptureLostCore(source);
+        }
+
+        private void CaptureLostCore(CaptureSource source)
         {
             if (Captured != null)
-                Capture(null, CaptureSource.Platform);
+                CaptureCore(null, source);
+            CaptureGestureRecognizerCore(null);
+            IsGestureRecognitionSkipped = false;
         }
 
         public void Capture(IInputElement? control)
@@ -52,6 +70,17 @@ namespace Avalonia.Input
         }
 
         internal void Capture(IInputElement? control, CaptureSource source)
+        {
+            if (_disposed)
+            {
+                Debug.Assert(control is null, "Capturing a pointer that no longer exists.");
+                return;
+            }
+
+            CaptureCore(control, source);
+        }
+
+        private void CaptureCore(IInputElement? control, CaptureSource source)
         {
             var oldCapture = Captured;
             var oldSource = CaptureSource;
@@ -148,7 +177,14 @@ namespace Avalonia.Input
 
         public void Dispose()
         {
-            // callers are responsible for calling Capture(null, source) with an appropriate source
+            if (_disposed)
+                return;
+
+            // Mark the pointer gone first, so a capture lost handler can't capture it again.
+            // It no longer exists, so the platform is the only source the release can come from.
+            _disposed = true;
+
+            CaptureLostCore(CaptureSource.Platform);
         }
 
         /// <summary>
@@ -156,6 +192,17 @@ namespace Avalonia.Input
         /// </summary>
         /// <param name="gestureRecognizer">The gesture recognizer.</param>
         internal void CaptureGestureRecognizer(GestureRecognizer? gestureRecognizer)
+        {
+            if (_disposed)
+            {
+                Debug.Assert(gestureRecognizer is null, "Capturing a pointer that no longer exists to a gesture recognizer.");
+                return;
+            }
+
+            CaptureGestureRecognizerCore(gestureRecognizer);
+        }
+
+        private void CaptureGestureRecognizerCore(GestureRecognizer? gestureRecognizer)
         {
             if (CapturedGestureRecognizer != gestureRecognizer)
             {

src/Avalonia.Base/Input/TouchDevice.cs

@@ -109,20 +109,14 @@ namespace Avalonia.Input
                     {
                         target.RaiseEvent(e);
                     }
-                    pointer?.Capture(null, CaptureSource.Implicit);
+                    pointer?.CaptureLost(CaptureSource.Implicit);
                 }
             }
 
             if (args.Type == RawPointerEventType.TouchCancel)
             {
                 _pointers.Remove(args.RawPointerId);
-                using (pointer)
-                {
-                    pointer?.Capture(null, CaptureSource.Platform);
-                    pointer?.CaptureGestureRecognizer(null);
-                    if (pointer != null)
-                        pointer.IsGestureRecognitionSkipped = false;
-                }
+                pointer?.Dispose();
             }
 
             if (args.Type == RawPointerEventType.TouchUpdate)
@@ -165,7 +159,7 @@ namespace Avalonia.Input
         internal void PlatformCaptureLost()
         {
             foreach (var pointer in _pointers.Values)
-                pointer.Capture(null);
+                pointer.PlatformCaptureLost();
         }
     }
 }

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -987,8 +987,8 @@ namespace Avalonia.Win32.Interop
             POINTER_FLAG_FOURTHBUTTON = 0x00000080,
             POINTER_FLAG_FIFTHBUTTON = 0x00000100,
             POINTER_FLAG_PRIMARY = 0x00002000,
-            POINTER_FLAG_CONFIDENCE = 0x00000400,
-            POINTER_FLAG_CANCELED = 0x00000800,
+            POINTER_FLAG_CONFIDENCE = 0x00004000,
+            POINTER_FLAG_CANCELED = 0x00008000,
             POINTER_FLAG_DOWN = 0x00010000,
             POINTER_FLAG_UPDATE = 0x00020000,
             POINTER_FLAG_UP = 0x00040000,

src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs

@@ -583,7 +583,6 @@ namespace Avalonia.Win32
                         break;
                     }
                 case WindowsMessage.WM_POINTERLEAVE:
-                case WindowsMessage.WM_POINTERCAPTURECHANGED:
                     {
                         if (!_wmPointerEnabled)
                         {
@@ -594,6 +593,17 @@ namespace Avalonia.Win32
                         e = CreatePointerArgs(device, timestamp, eventType, point, modifiers, info.pointerId);
                         break;
                     }
+                case WindowsMessage.WM_POINTERCAPTURECHANGED:
+                    {
+                        if (!_wmPointerEnabled)
+                        {
+                            break;
+                        }
+                        GetDevicePointerInfo(wParam, out var device, out var info, out var point, out var modifiers, ref timestamp);
+                        var eventType = device is TouchDevice ? RawPointerEventType.TouchCancel : RawPointerEventType.CancelCapture;
+                        e = CreatePointerArgs(device, timestamp, eventType, point, modifiers, info.pointerId);
+                        break;
+                    }
                 case WindowsMessage.WM_POINTERWHEEL:
                 case WindowsMessage.WM_POINTERHWHEEL:
                     {
@@ -1300,7 +1310,7 @@ namespace Avalonia.Win32
             var isTouch = info.pointerType == PointerInputType.PT_TOUCH;
             if (info.pointerFlags.HasFlag(PointerFlags.POINTER_FLAG_CANCELED))
             {
-                return isTouch ? RawPointerEventType.TouchCancel : RawPointerEventType.LeaveWindow;
+                return isTouch ? RawPointerEventType.TouchCancel : RawPointerEventType.CancelCapture;
             }
 
             var eventType = ToEventType(info.ButtonChangeType, isTouch);

tests/Avalonia.UnitTests/MouseTestHelper.cs

@@ -101,9 +101,7 @@ namespace Avalonia.UnitTests
                 else
                     target.RaiseEvent(e);
 
-                _pointer.Capture(null);
-                _pointer.CaptureGestureRecognizer(null);
-                _pointer.IsGestureRecognitionSkipped = false;
+                _pointer.CaptureLost(CaptureSource.Explicit);
             }
             else
                 Move(target, source, position ?? default);

tests/Avalonia.UnitTests/TouchTestHelper.cs

@@ -64,9 +64,7 @@ namespace Avalonia.UnitTests
 
         public void Cancel()
         {
-            _pointer.Capture(null);
-            _pointer.CaptureGestureRecognizer(null);
-            _pointer.IsGestureRecognitionSkipped = false;
+            _pointer.CaptureLost(CaptureSource.Platform);
         }
     }
 }