[Webkit-unassigned] [Bug 236312] New: Scroll values misreported/regressed (bug)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=236312

            Bug ID: 236312
           Summary: Scroll values misreported/regressed (bug)
           Product: WebKit
           Version: Safari 15
          Hardware: iPhone / iPad
                OS: Unspecified
            Status: NEW
          Severity: Major
          Priority: P2
         Component: Scrolling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jack at greensock.com
                CC: simon.fraser at apple.com

Set the scroll position via JavaScript and then immediately check it and it seems to work correctly but under heavy load it actually REVERTS to a previous scroll position on the next requestAnimationFrame()! From what I can tell, it's definitely a bug. Here is the proof (you must run this on iOS Safari in the /debug/ window of CodePen so that it's not in an iframe): https://codepen.io/GreenSock/pen/16c435b12ef09c38125204818e7b45fc?editors=0010

Again, this only happens under heavy load. 

I understand JS runs on a separate thread from scrolling but all of this is happening in JS world so the regression of the scroll position doesn't make sense to me. 

Can anyone explain why this is happening and how we can work around it?

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