[Webkit-unassigned] [Bug 233137] New: `gestureend` event fired twice for same gesture, occasionally

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=233137

            Bug ID: 233137
           Summary: `gestureend` event fired twice for same gesture,
                    occasionally
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: UI Events
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dan at moqups.com

Created attachment 444274

  --> https://bugs.webkit.org/attachment.cgi?id=444274&action=review

Logger that displays wheel, touch, and gesture events

Occasionally when performing a pinch & twist gestures (that produce, in turn, a scale and a rotation in the emitted gesture events), the `gestureend` event is fired twice for the same gesture.

This goes against the expectation in regards to the order of gesture events (a series of `gesturechange` events bookended by exactly one `gesturestart` and exactly one `gestureend` event). It can also cause problems where the `gestureend` handler is not idempotent, and in the case of applying transformations, the handler is expected not to be.

The bug is elusive, but can hopefully be caught in the attached test file (which I believe is correct, unless I'm missing something obvious). I will also attach the bug repro in video form.

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