[Webkit-unassigned] [Bug 212640] New: getClientBoundingRect values in reQuestAnimationFrmae out of date compared to painted appearance

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=212640

            Bug ID: 212640
           Summary: getClientBoundingRect values in reQuestAnimationFrmae
                    out of date compared to painted appearance
           Product: WebKit
           Version: Safari 13
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ian at ianbellomy.com
                CC: dino at apple.com

Created attachment 400831

  --> https://bugs.webkit.org/attachment.cgi?id=400831&action=review

Repro / example

The values returned by getClientBoundingRect (gCBR) of an element with a transform transition, when used in requestAnimationFrame (rAF), appear to describe what is presented rather than what will be painted post rAF. 

But in Chrome and FF, gCBR values consistently match what _will be_ painted, such that gCBR can be used to perfectly align elements to other transitioning elements (see attached example in Safari and Chrome).

Reading the specs for getClientBoundingRect and requestAnimationFrame, I can't yet tell whether the data in this situation is _supposed_ to agree with what is on screen or what _will_ be on screen. The inconsistency is a paint point though.

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