[fullscreen] Propose to always render elements in the top layer as if it is fixed positioned (#19)

The current spec explicitly states that elements in the top layer may have `position: absolute` and in which case, it's containing block would be the initial containing block.

If I understand correctly, the difference for the containing block to be the viewport and the initial containing block is whether the position is affected by the scroll offset. However, does it make sense to allow scroll offset to affect the position of a top layer element? I don't think so.

The top layer elements should be rendered on top of the page scrollbars, otherwise it would be really unfortunate for fullscreen.

If we position something in the top layer based on the scroll offset, and it is shown during fullscreen, it could easily be positioned in a place where user can never see, because user is not able to operate the scrollbar of the page in fullscreen in general.

Thus I propose that we always use the viewport as the containing block of the elements in the top layer, and do not touch the `position` property at all.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fullscreen/issues/19