Backdrop-filter should establish a containing block, and be a CSS stacking context.
<rdar://problem/115638583>
Pull request: https://github.com/WebKit/WebKit/pull/17887
Committed 268161@main (0f4cd6871883): <https://commits.webkit.org/268161@main> Reviewed commits have been landed. Closing PR #17887 and removing active labels.