We can avoid some repaint work when the repaint is clipped out at an iframe boundary.
<rdar://problem/118024306>
Pull request: https://github.com/WebKit/WebKit/pull/20073
Committed 270307@main (94b07281b697): <https://commits.webkit.org/270307@main> Reviewed commits have been landed. Closing PR #20073 and removing active labels.