Created attachment 465337 [details] Screencast showing top bar of scmp.com while site is scrolled On scmp.com, the top bar of the site is jumpy and glitchy when the website is scrolled. Reproducible on any WebKitGTK.
I can reproduce with 2.38 too, but it's maybe a bit less noticeable.
Also reproducible on WPE, and on Mac, in Safari Technology Preview 164 and on trunk. Not reproduced in stable Safari 16.1.
position:fixed inside overflow scroll. Importantly, it's jittery even when JavaScript is disabled, which means it's a scrolling tree bug.
This didn't regress in 2.39.90 and it's cross-platform issue.
<rdar://problem/106407870>
The page has a “position:fixed” element inside overflow:scroll, but with a transformed ancestor, so the “position:fixed” behaves more like “position:absolute”. But for some reason we don’t reposition it on the scrolling thread correctly.
I think we need to educate the scrolling tree about fixed elements that don’t behave like fixed (because of an ancestor transform), but whose containing block is still the viewport.
Reverting 247609@main fixes this.
Pull request: https://github.com/WebKit/WebKit/pull/11955
Committed 262118@main (cf604717c41c): <https://commits.webkit.org/262118@main> Reviewed commits have been landed. Closing PR #11955 and removing active labels.