[Webkit-unassigned] [Bug 195388] Fixed elements can flicker on a page that is busy doing layout

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=195388

--- Comment #1 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
Seen on macOS, reproduces in shipping Safari.

I think what's happening is that the main thread can be busy running JS etc, allowing a  commit on the main thread with a very stale position for the fixed layer. Meanwhile, the scrolling thread has moved ahead, and is continually pushing the layer to its correct position.

The fix is to allow the scrolling tree to have the final say on layer positions, so post-flush, traverse the scrolling tree on the main thread and have it set positions based on the scrolling thread's scroll position.

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