fullscreen

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@annevk says, we should not be propagating the background to the viewport in full screen. I ran into this problem in firefox when reveal.js full screened the body and lost my background image. Changing reveal.js to full screen the documentElement instead of the body avoided the problem.

@annevk thinks this is a spec issue.

In particular, the propagation from HTML body to the viewport (if root doesn't have a background), or root to the viewport, should probably not happen in fullscreen. Otherwise ::backdrop will hide it. It seems that browsers don't propagate for root if it becomes fullscreen, but do for HTML body.

@tabatkins @fantasai thoughts?

Note that fullscreen root doesn't have a visible ::backdrop pseudo-element, so it doesn't matter whether the background is propagated.

Also note that a fullscreen body won't have the document level scrollbar because it is in general just a normal block element. So if you want to fullscreen the whole page, you should always request fullscreen on root. I don't think we want to make body any more special.

Hmm, so why would fullscreen root not have a visible ::backdrop?

That is #24.

@upsuper can you still style ::backdrop for root? We didn't really address that there it seems.

That's a... good question. In Gecko's implementation, you cannot, but the spec seems to indicate that you should be able to... I guess I can make Gecko behave that way...