Fix pointer capture cancellation on Win32 by kekekeks · Pull Request #22054 · AvaloniaUI/Avalonia

kekekeks · 2026-08-24T15:05:24Z

This PR has separate connected changes:

hardened Pointer class:
a) Dispose now properly releases any dangling captures and prevents pointer from being captured again
b) Gesture recognizer captures are now correctly released, platforms are required to call CaptureLost(source) instead of Capture(null) which doesn't affect recognizer capture.

E2E-ish test coverage will arrive with headless pen/touch PR.

fix for cancelled pen (e. g. Ctrl+Alt+Del, Win+L) interaction was reported as PointerUp because POINTER_FLAG_CANCELED was invalid due to what I assume to be a typo.

WM_POINTERCAPTURECHANGED and previously mishandled POINTER_FLAG_CANCELED now map to CancelCapture.

PenDevice handles CancelCapture just as cancelled capture, pointer is not dropped until actual WM_POINTERLEAVE/LeaveWindow.

Non-win32 backends are not yet updated, the only observable change is that any (invalid) leftover capture is cleared when pointer is disposed in LeaveWindow.

followup plan:

test coverage of x-plat code as via pen/touch headless PR
update platform backends

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)

avaloniaui-bot · 2026-08-24T15:49:02Z

You can test this PR using the following package version. 12.2.999-cibuild0068693-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

Copilot

Pull request overview

This PR hardens pointer capture lifecycle handling and fixes Win32 pen/touch cancellation/capture-change mapping so that cancellations end capture correctly (instead of being misreported as pointer-up/leave).

Changes:

Add a centralized Pointer.CaptureLost(source) path and make Pointer.Dispose() clear dangling captures (including gesture-recognizer capture).
Update Win32 message handling to map WM_POINTERCAPTURECHANGED and POINTER_FLAG_CANCELED to CancelCapture/TouchCancel as appropriate.
Update mouse/pen/touch devices and unit test helpers to use CaptureLost(...) rather than manually clearing multiple pointer fields.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/Avalonia.UnitTests/TouchTestHelper.cs	Switch test helper cancellation to the new `CaptureLost(...)` API.
tests/Avalonia.UnitTests/MouseTestHelper.cs	Switch test helper capture release to `CaptureLost(...)`.
src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs	Map Win32 pointer capture-change/canceled-flag to cancel-capture semantics.
src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs	Correct `PointerFlags` bit values for confidence/canceled.
src/Avalonia.Base/Input/TouchDevice.cs	Use `CaptureLost(...)`/`Dispose()` for touch end/cancel cleanup.
src/Avalonia.Base/Input/Pointer.cs	Add disposed-guarding and the unified `CaptureLost(...)` capture teardown path.
src/Avalonia.Base/Input/PenDevice.cs	Handle `CancelCapture` without dropping the pointer until leave, and use `CaptureLost(...)` on pen up.
src/Avalonia.Base/Input/MouseDevice.cs	Use `CaptureLost(...)` on mouse up and honor cancel-capture events.

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avaloniaui-bot · 2026-08-24T18:24:58Z

You can test this PR using the following package version. 12.2.999-cibuild0068721-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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)

avaloniaui-bot · 2026-08-25T14:40:15Z

You can test this PR using the following package version. 12.2.999-cibuild0068792-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

* 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) * 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) --------- Co-authored-by: Claude Opus 5 (1M context)

MrJul added bug backport-candidate-12.1.x Consider this PR for backporting to 12.1 branch os-windows labels Aug 24, 2026

kekekeks marked this pull request as ready for review August 24, 2026 17:21

kekekeks requested a lite review from Copilot August 24, 2026 17:21

Copilot started reviewing on behalf of kekekeks August 24, 2026 17:22 View session

Copilot AI reviewed Aug 24, 2026

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Comment thread src/Avalonia.Base/Input/Pointer.cs

Comment thread tests/Avalonia.UnitTests/TouchTestHelper.cs

Comment thread tests/Avalonia.UnitTests/MouseTestHelper.cs

MrJul self-assigned this Aug 25, 2026

MrJul previously approved these changes Aug 25, 2026

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kekekeks dismissed MrJul’s stale review via a4cc301 August 25, 2026 13:39

kekekeks force-pushed the fixes/win32-pen-pointer-capture-lost branch from f9d049c to a4cc301 Compare August 25, 2026 13:39

kekekeks force-pushed the fixes/win32-pen-pointer-capture-lost branch from a4cc301 to a162279 Compare August 25, 2026 13:47

MrJul approved these changes Aug 25, 2026

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MrJul enabled auto-merge August 25, 2026 13:50

MrJul added this pull request to the merge queue Aug 25, 2026

Merged via the queue into main with commit a873896 Aug 25, 2026
9 checks passed

MrJul deleted the fixes/win32-pen-pointer-capture-lost branch August 25, 2026 15:23

zrt2399 mentioned this pull request Aug 26, 2026

Regression: RefreshContainer cannot be refreshed by pull gesture #22075

Closed

kekekeks mentioned this pull request Aug 26, 2026

Use _effective_ capturer (which can be recognizer) for PlatformCapture #22076

Merged

MrJul added backported-12.1.x and removed backport-candidate-12.1.x Consider this PR for backporting to 12.1 branch labels Sep 2, 2026

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Fix pointer capture cancellation on Win32 - #22054

Fix pointer capture cancellation on Win32#22054
MrJul merged 2 commits into
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fixes/win32-pen-pointer-capture-lost

kekekeks commented Aug 24, 2026 •

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kekekeks commented Aug 24, 2026 •

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