[css-scroll-snap] choose closet snap target if neither are visible
<rdar://problem/99897147>
Pull request: https://github.com/WebKit/WebKit/pull/4336
Committed 254982@main (0efae6b49e5c): <https://commits.webkit.org/254982@main> Reviewed commits have been landed. Closing PR #4336 and removing active labels.