An implicit-root IntersectionObserver should be able to have targets in multiple windows, and observe the intersection of each of them with their window's viewport.
Created attachment 354579 [details]
Attachment 354579 [details] did not pass style-queue:
ERROR: Source/WebCore/ChangeLog:8: You should remove the 'No new tests' and either add and list tests, or explain why no new tests were possible. [changelog/nonewtests] 
Total errors found: 1 in 7 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Created attachment 354580 [details]
Comment on attachment 354579 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=354579&action=review
> + No new tests (OOPS!).
(In reply to Simon Fraser (smfr) from comment #4)
> Comment on attachment 354579 [details]
> View in context:
> > Source/WebCore/ChangeLog:8
> > + No new tests (OOPS!).
> Fix this.
Removed that line in the latest patch since there's a web platform test that covers this (and passes after this patch).
Is that sufficient test coverage or do you think we need more?