[css-contain-3] visibility/content-visibility inconsistency regarding dialogs

[rwlbuis]

In #6939 (comment) a change was made to set visibility: visible on modal dialogs. This overwrites any visibility set on a parent element, so visibility: hidden on a parent would not hide the modal dialog.

For content-visibility however the expectation is different. This test verifies that content-visibility: hidden hides the complete subtree, including the modal dialog:
https://github.com/web-platform-tests/wpt/blob/830ae49918/css/css-contain/content-visibility/content-visibility-with-top-layer-002.tentative.html

To me this feels inconsistent and potentially confusing to web developers. Thoughts?

[tabatkins]

I don't think the two properties necessarily have anything to do with each other, similarity in name notwithstanding.

That said, consistency with my position in January suggests that c-v:hidden shouldn't have an effect on something in the top layer, as its position in the box tree no longer sees a hidden ancestor.

(We had to add special carve-outs for visibility and pointer-events because they're inherited properties evaluated on the element itself, rather than non-inherited properties that apply an effect to a subtree.)

[vmpstr]

We've also resolved separately (#6728) to say that elements in c-v hidden subtrees will not have boxes in the top layer, since that can cause performance problems (ie style is not updated on the element because of c-v hidden ancestor but that ancestor isn't there in the layout tree).

[nt1m]

I do think it is a bit confusing that visibility has a special carve out and that content-visibility doesn't, perhaps we should undo #6939 (comment) ? Right now only WebKit/Safari implements this resolution, Firefox & Chrome haven't gotten around to it yet.

[tabatkins]

Like I said, the carve-outs exist solely because those properties are inherited (and thus determine whether or not they apply solely on the element tree, which is unaffected by top-layer, since that only affects the box tree). The two properties are completely unrelated other than the fact that they both have the word 'visibility' in their name.