[cssom-view] Allow elementFromPoint and elementsFromPoint to traverse shadow roots

I recently asked on Mastodon if folks knew the element(s)FromPoint() methods and whether they'd want them to be able to traverse shadow roots: https://mas.to/@patrickbrosset/113236606097090887

Some replies to my "Do you wish it would across Shadow DOM boundaries "question:

In my use case, whether I want it to cross Shadow DOM boundaries would depend on how "thick" the custom elements are. If thin, I wouldn't care. If they encompassed too much in the page, then yeah I'd want a more fine-grained approach.

YES

ABSOLUTELY YES

I'm not sure whether it should cross shadow boundaries on its own. I could maybe see an option, so you had to do work to get through, but didn't need to write your own recursive method.

I think I’d want to be explicit about getting any element from within a shadow root, so maybe if there was an option to include shadow roots.

Absolutely yes! Eventually, it could be a parameter option, like document.elementsFromPoint({ composed: true }) or similar.

Note that caretPositionFromPoint already works this way: https://developer.mozilla.org/en-US/docs/Web/API/Document/caretPositionFromPoint#shadowroots

Also, during the TAG review for caretPositionFromPoint @LeaVerou said (https://github.com/w3ctag/design-reviews/issues/949#issuecomment-2186090703):

I would reiterate that elementFromPoint() could also be expanded in the same way for consistency (and I suspect there are plenty of use cases for that too).

Start with the CSSOM View issue discussion and review the existing caretPositionFromPoint shadow-root behavior linked in the report. Read the TAG review for the related consistency rationale. Done would require an agreed API design and corresponding specification changes, but this issue does not identify files or tests.