Re: [fullscreen] Styles of former fullscreen elements (#18)

Almost everything has changed, doesn't it?

Now we do not stop fullscreen request for elements which are not descendant of the current fullscreen element, which means the inconsistency between case 1 and case 2 no longer exists. Also we resolved to make the topmost top layer element obscure everything else, so the issue of case 3 doesn't exist any more, either.

In addition, at that time, the top layer and the fullscreen element stack are synchronous (when there is no `<dialog>`) which seems to be redundant, and is not what I'm proposing here. I propose that we should only put the topmost element of fullscreen element stack to the top layer, instead of all of them.

The issue is the style. In the spec, value of `position` other than `fixed` and `absolute` will be computed to `absolute` for top layer elements, while we force `fixed` to `:fullscreen`. Also the geometry dimensions are weird that, elements in the top layer has `left`, `right`, `top` set to 0, while we set all four dimensions to 0 for `:fullscreen`.

Obscuring things only means that they won't get focus, not that they are invisible, nor they are not accessible from the script. Generally, the elements are not visible, but authors may make the fullscreen element as well as the `::backdrop` pseudo-element transparent to uncover the things underneath. It could also happen when one of the ancestors of the fullscreen element or the element itself has `display: none`. Moreover, the script may also want to get the bounding rect of the elements in question.

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https://github.com/whatwg/fullscreen/issues/18#issuecomment-134130500