c.f. https://wpt.fyi/results/css/CSS2/stacking-context/composite-change-after-scroll-preserves-stacking-order.html?label=master&product=chrome%5Bexperimental%5D&product=firefox%5Bexperimental%5D&product=safari-171%20preview%5Bexperimental%5D&product=safari-172%20preview&product=safari%5Bstable%5D bisect points at 259592@main as the culprit we don't have this imported in WebKit (bug 209735), see http://wpt.live/css/CSS2/stacking-context/composite-change-after-scroll-preserves-stacking-order.html
<rdar://problem/110902526>