[css-anchor-position-1] anchors-visible still needs more details

[emilio]

In #12732 we resolved that position-visibility: anchors-visible used the same mechanics as IntersectionObserver, as if you used IntersectionObserver with the abspos cb as root.

That all seems fine, however when trying to implement it to the letter there are a few tests that highlight somewhat diverging behavior and need more clarification.

• Even the basic test (https://wpt.fyi/results/css/css-anchor-position/position-visibility-anchors-visible.html) would not pass with the .isIntersecting definition from IntersectionObserver because that uses edge-inclusive intersection (https://w3c.github.io/IntersectionObserver/#update-intersection-observations-algo), so when you're right at the edge you'd claim the anchor is visible. This can probably be clarified in the anchor positioning spec by additionally requiring a non-zero intersectionArea.

• When both anchor and anchored are in the same containing-block (siblings) apparently anchors-visible is not intended to apply. position-visibility-anchors-visible-both-position-fixed.html tests it. I'm not sure this is warranted, and I think this should probably change.

cc @tabatkins @fantasai

[emilio]

The intersection area thing doesn't quite work because it trips for zero-height / width anchors.