[Webkit-unassigned] [Bug 222132] New: Sticky/fixed layers that are resized on the main thread can appear stretched when scrolling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=222132

            Bug ID: 222132
           Summary: Sticky/fixed layers that are resized on the main
                    thread can appear stretched when scrolling
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Compositing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simon.fraser at apple.com
                CC: simon.fraser at apple.com

Scroll-coordinated layers, for fixed and sticky, get their CALayer's position changed on the scrolling thread. However, if the main thread also commits a size change, but the layer contents are slow to paint, then CA can do the size change on the scrolling thread before the contents are painted, resulting in stretched content. This happens on https://www.apple.com/ipad-air/ in low memory configurations.

The fix is to use another layer as the "anchor" for fixed and sticky.

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