[UI-side compositng] Flashing scrollbars causes the scrollbar to never hide
<rdar://problem/106329824>
Pull request: https://github.com/WebKit/WebKit/pull/11140
Committed 261333@main (d9636c006aee): <https://commits.webkit.org/261333@main> Reviewed commits have been landed. Closing PR #11140 and removing active labels.