[Webkit-unassigned] [Bug 222939] New: [Web Animations] setKeyframes does not preserve animation's current offset

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=222939

            Bug ID: 222939
           Summary: [Web Animations] setKeyframes does not preserve
                    animation's current offset
           Product: WebKit
           Version: Safari 14
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ldebeasi at gmail.com
                CC: dino at apple.com, graouts at apple.com

Created attachment 422613

  --> https://bugs.webkit.org/attachment.cgi?id=422613&action=review

Code reproduction

When using setKeyframes() on an animation, WebKit does not preserve the animation's current offset. For example, if an animation is at 100% progress and setKeyframes() is called, WebKit will reset the animation's progress to 0%.

Steps to reproduce:

1. Open attached code reproduction in either Safari for iOS or macOS.
2. Observe that a blue square translates from left to right and then stops.
3. Press "Update Keyframes". You should notice that the square jumps back to the left/starting position.

Expected Behavior:

I would expect that after pressing "Update Keyframes" that the square stays at its ending position.

Actual Behavior:

The square is reset back to its starting position.

Other Information:

- I can reproduce this in STP 121 and iOS 14.5, though I can reproduce this on stable releases as well.
- Both Chrome and Firefox preserve the animation's current offset as expected.
- This is only an issue if "fill: 'both'" is set on the animation.

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