http://fullscreen.spec.whatwg.org/#dom-element-requestfullscreen

fullscreenElements is populated synchronously, but before the animation frame task is run, the elements of fullscreenElements may have been moved in ways that still passes the ready checks, but leaves the resulting fullscreen element stacks inconsistent, for example if the pending element is moved between two iframes immediately after requestFullscreen() returns.

The fix should be to populate the list of newly fullscreen elements in the animation frame task, immediately before pushing to stacks and firing events.

Note that in such a rewrite, the ready check could be performed for only the pending element, since it is now recursive.