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The spec says the containing block of an element in the top layer is the initial containing block. However, this would mean elements with fixed positioning don't behave as such: they would be positioned relative to the document's origin rather than the viewport's. Instead, the containing block of fixpos elements should be the viewport, as normal.
Ian and Robert came up with this if I remember correctly. I'd like them to comment here first.
To be clear, comment 0 is about http://fullscreen.spec.whatwg.org/#rendering which should eventually move into CSS.
I don't think it was me. However, viewport sounds right.
So, instead of "Its containing block is the initial containing block." we should have "If its position property computes to fixed, its containing block is the viewport, and the initial containing block otherwise."
comment 4 looks good to me.
Thanks! https://github.com/whatwg/fullscreen/commit/d77e0158a10a2e1904ade9eac6d03531437c3ef7