Right now, there doesn't seem to be any leeway in the spec for the user agent to unilaterally decide to end fullscreen for an element.

I know that we do this already in Chromium: if a page spawns a pop-up window, we drop fullscreen. I'm arguing in a Blink Intent that the same should be done for when a page spawns a dialog. But whatever your position is on that particular Intent, surely the ability for a UA to decide that in certain situations, it needs to force an end to fullscreen needs to be there.

@foolip suggested in a post as one of a list of options:

Something more handwavy in https://fullscreen.spec.whatwg.org/#ui giving the UA a license to exit whenever it thinks it's in the best interest of the user.

The other option was a specific one about the Intent, but I think this wider option is appropriate.