Hide all non-fullscreen elements when in fullscreen mode
rdar://problem/18912707
Does this destroy page state in a way that pages will detect (e.g. plug-in state)?
Pages could detect state changes via window.getComputedStyle(), but this is already true of existing fullscreen UA styles. I don't know if destroying a plugin renderer by setting display:none will destroy that plug-in's state.
Would using 0 opacity be less destructive?
Opacity:0 would not be enough. The elements would still get renderers, would still participate in layout, and could still force the backing store of their ancestors to become larger than the fullscreen element.