Somehow https://commits.webkit.org/263699@main didn't contain the most crucial code change to make performPostLayoutTasks not trigger synchronous scroll or focus.
Pull request: https://github.com/WebKit/WebKit/pull/13485
Committed 263724@main (be4701c3c0d1): <https://commits.webkit.org/263724@main> Reviewed commits have been landed. Closing PR #13485 and removing active labels.
<rdar://problem/108957029>
This caused bug 257576.