As noted by @upsuper in #87 (comment):

when this happens, the browser window would end up being in fullscreen state while the content is not?

A test which could be adapted to cover this is https://github.com/w3c/web-platform-tests/blob/5e1a3b80cea8d36774d2afd78b29a74792e9f15a/fullscreen/api/element-request-fullscreen-and-move-to-iframe-manual.html

Invoking "exit fullscreen" wouldn't be enough to recover, because it is a no-op when there is no fullscreen element. Rather, 'resize something's viewport to its "normal" dimensions' would be needed.