As in the summary - perhaps it's intentional that scrollbars can appear outside of the content they correspond to, but this is causing some tests to fail with async overflow scroll enabled; namely imported/w3c/web-platform-tests/css/css-flexbox/overflow-auto-001.html (and perhaps more that we'll find out via EWS/bots if this lands)
Created attachment 430970 [details] Patch
Created attachment 431325 [details] Patch
Comment on attachment 431325 [details] Patch The 1 failure on Mac is because the expectations don't expect the overflow container layer to clip its children (which is what this patch modifies) - assuming the approach is deemed correct, I'll modify the expectations.
Created attachment 431419 [details] Patch
Created attachment 431420 [details] Patch
Comment on attachment 431420 [details] Patch Added a test for this in compositing/scrolling/async-overflow-scrolling as the only test that was hitting this was in WPT.
Committed r278883 (238825@main): <https://commits.webkit.org/238825@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 431420 [details].
<rdar://problem/79351898>