[Webkit-unassigned] [Bug 161535] New: Implement the transitioncancel event

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=161535

            Bug ID: 161535
           Summary: Implement the transitioncancel event
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: webkit at chrisrebert.com
                CC: dino at apple.com

Specification: https://drafts.csswg.org/css-transitions-2/#eventdef-transitionevent-transitioncancel

Steps to reproduce the problem:
1. Open the CSSWG testcase https://github.com/w3c/csswg-test/blob/master/css-transitions-2/transitioncancel-001.html in Safari.

What is the expected behavior?
The test should pass.

What went wrong?
The test failed, indicating that Safari is not firing the transitioncancel event when a transition is canceled.
(In this case, the cancellation is due to the element being made display:none; while the transition was in progress.)


Without transitioncancel, scripts which are waiting for the end (whether normal or canceled early) of a CSS transition have to resort
to using setTimeout (or similar) to ensure that their callbacks still get called even if the transition gets canceled.
This extra complexity is annoying to authors, and frameworks often include such setTimeout-based workarounds in practice, for example:
* https://github.com/twbs/bootstrap/blob/c56219d223af5145c171defb9fa1426e1dd022f3/js/transition.js#L36
* https://github.com/Semantic-Org/Semantic-UI/blob/f725b162e70896e38257965424ac7f9af486b927/src/definitions/modules/transition.js#L441
Such extra setTimeout()s also presumably have a negative impact on performance.

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