Bug 322698 – [GTK][WPE] touchmove is dropped when it arrives while touchstart is still in flight

• Status: RESOLVED
• Resolution: FIXED
• Component: UI Events

[Pablo Saavedra]

[GTK][WPE] touchmove is dropped when it arrives while touchstart is still in flight

[Pablo Saavedra]

Pull request: https://github.com/WebKit/WebKit/pull/72574

[Pablo Saavedra]

Regression due to `2dce2395a46b` *[GTK][WPE] Support touch event asynchronous scrolling* (bug 318938, `319347@main`, Fujii Hironori, 2026-08-17) in a downstream webdriver test:


Test Code excerpt:

``` python
        # Check touch move
        expected_scroll_position = 300
        pointer_action = action.pointer_action
        pointer_action.pointer_down()
        pointer_action.move_by(10, -expected_scroll_position)
        pointer_action.pointer_up()
        action.perform()
        logged_events = driver.execute_script("return window.loggedEvents;")
        expected_events = ['touchstart', 'touchend', 'mouseover',
                           'mousemove', 'mousedown', 'mouseup',
                           'click', 'touchstart', 'touchmove',
                           'mouseout', 'mouseover', 'mousemove',
                           'touchend']
        assert logged_events == expected_events, \
               f"Expectations not satisfied {logged_events} " \
               f"!= {expected_events}"

        scroll_position = driver.execute_script("return window.pageYOffset;")
        assert scroll_position == expected_scroll_position, \
               f"Expectations not satisfied. Y-Scroll {scroll_position} " \
               f"!= {expected_scroll_position} (expected)"
```


## Failure

Assertion failed: Expectations not satisfied:

* Expected:      ['touchstart', 'touchend', 'mouseover', 'mousemove', 'mousedown', 'mouseup', 'click', 'touchstart', 'touchmove', 'mouseout', 'mouseover', 'mousemove', 'touchend']
* Actual result: ['touchstart', 'touchend', 'mouseover', 'mousemove', 'mousedown', 'mouseup', 'click', 'touchstart', 'touchend', 'mousemove', 'mousedown', 'mouseup', 'click']

The first tap matches expectations; the divergence starts at the 8th event, where `touchmove` is missing and the sequence degenerates into a duplicated tap.


## Cause

The commit added a serialized UI-process touch queue for the new `ENABLE(COORDINATED_TOUCH_EVENTS)` path but the wrong mechanism (async-scrolling/event-region coalescing). The real cause is a plain missing `else`: in the  code at `Source/WebKit/UIProcess/WebPageProxy.cpp:5566`:

```cpp
if (event.type() == WebEventType::TouchMove && !internals().touchEventQueue.isEmpty()) {
    QueuedTouchEvents& lastEvent = internals().touchEventQueue.last();
    if (lastEvent.forwardedEvent.type() == WebEventType::TouchMove)
        lastEvent.deferredTouchEvents.append(event);
    // <-- no else: event is silently discarded
} else {
    internals().touchEventQueue.append(event);
    if (internals().touchEventQueue.size() == 1)
        processNextQueuedTouchEvent();
}
```

When a `TouchMove` arrives while the queue is non-empty **but the last queued event is not itself a `TouchMove`**;  i.e. while the `TouchStart` is still awaiting its async reply from the web process — neither branch runs. The event is neither forwarded to the web process nor appended to `deferredTouchEvents`. It is dropped.

Because `PageClientImpl::doneWithTouchEvent()` is only reached for events that made it into the queue, a dropped `TouchMove` is lost twice: the DOM never sees `touchmove`, and the WPE gesture controller never sees the motion either. The gesture controller then classifies down+up-with-no-motion as a tap and synthesizes `mousemove`/`mousedown`/`mouseup`/`click`. Nothing scrolls, so the test's second assertion (`pageYOffset == 300`) would also have failed.


`WebAutomationSession::platformSimulateTouchInteraction()` (`Source/WebKit/UIProcess/Automation/libwpe/WebAutomationSessionWPE.cpp:527`) emits exactly **one** `WPE_EVENT_TOUCH_MOVE` per move action, with no interpolation and no wait — there is an explicit TODO for this (https://bugs.webkit.org/show_bug.cgi?id=275031). So the sequence is:

1. `TOUCH_DOWN`-> queue empty -> appended, size 1 -> sent to web process, reply pending.
2. `TOUCH_MOVE` -> queue non-empty, last is `TouchStart` -> **dropped**.
3. `TOUCH_UP` -> not a move -> appended normally.

[EWS]

Committed 319960@main (f0c538173039): <https://commits.webkit.org/319960@main>

Reviewed commits have been landed. Closing PR #72574 and removing active labels.