After r279800 IntersectionObserver's callback may never be delivered if the observed element has been disconnected from the document, and the element has been explicitly unobserved by IntersectionObserver.
ResizeObserver has a similar code but ResizeObserver.unobserve active observation as well so it doesn't exhibit a bug per se. I don't think this is correct per spec though. Will investigate separately.
Created attachment 434696 [details] Fixes the bug
Comment on attachment 434696 [details] Fixes the bug r=me
<rdar://81057759>
Comment on attachment 434696 [details] Fixes the bug Clearing flags on attachment: 434696 Committed r280549 (240176@main): <https://commits.webkit.org/240176@main>
All reviewed patches have been landed. Closing bug.