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

Blink do A for `<dialog>`. For fullscreen, I believe all browsers currently still use `:root:fullscreen-ancestor { overflow: hidden; }`, which is the third way I mentioned.

However, we have removed `:fullscreen-ancestor` long ago. In addition, as we've also removed the hierarchy restriction, it now becomes hard to maintain that pseudo-class efficiently.

(Also, changing `overflow` property is expensive in Gecko, hence I want to get rid of it anyway)

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