Check for full visibility

[jsha]

We should have a check for "this element is fully visible," where "visible" means "fully in the viewport, and not obscured by other elements."

We might be able to do this with elem.getClientBoundingRect(), and checking (a) that the bounding rect is within the viewport, and (b) checking each corner of the element with https://developer.mozilla.org/en-US/docs/Web/API/Document/elementFromPoint, to ensure that it is the element returned (i.e. there's nothing stacked on top).

[GuillaumeGomez]

There could still be elements not covering the corners but still being over the element we're checking full visibility for. I think it's a very common problem for web GUI tests actually.