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Here's an idea that doesn't fix any bug but may be worthwhile. In requestFullscreen(), wait with the first ready check until just before the resize, so that if it requires IPC, that won't have to be done synchronously before requestFullscreen() returns. "Fullscreen is not supported" could be moved as well, just in case establishing that actually requires inspecting each document. Together with bug 26601 that should leave nothing fancy in the synchronous section. In exitFullscreen(), return immediately after step 1 or 2, or otherwise move the population of parentDocs to the async section. WDYT?
On the other hand, this change would require specifying in greater detail when exactly the async steps are run, so that it's well defined which scripts and event handlers run between the requestFullscreen() call and the first async steps. That might not be an improvement.
Not sure what to do here. Did you experiment with this?
I didn't experiment, unfortunately I'm just speculating on what might be a problem. Doing the cross-process checks synchronously in requestFullscreen() could turn out to be a mistake, but the check needs to be done before the actual resize at a well-defined point for interop, so I don't know how to improve it. Resolving as invalid until the problem turns out to be real.