[Webkit-unassigned] [Bug 179789] New: Style recomputation breaks animations that were paused on initial render

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=179789

            Bug ID: 179789
           Summary: Style recomputation breaks animations that were paused
                    on initial render
           Product: WebKit
           Version: Safari 11
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: aaron at sikes.io

Created attachment 327089

  --> https://bugs.webkit.org/attachment.cgi?id=327089&action=review

Reproduction

If animation-play-state is "paused" for a given animation on page load, and a style recomputation happens before it is changed to running, the animation properties no longer seem to apply to the element.

See this reproduction (I've also attached a simple html file): https://jsfiddle.net/6c63z10z/10/
I'm on macOS 10.12, but a colleague of mine said he sees the same issue in Safari 11 on high sierra.

In Chrome, Firefox, both boxes start spinning at once, 500ms after page load.

In Safari, one of two things will happen:
- At 500ms after page load, the first box will snap to horizontal, and at 1000ms, the right box will start to spin
- At 500ms after page load, both boxes will snap to horizontal, and neither will begin spinning

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