who only by moving can balance

is there any way to make it stay not-fullscreen? I'm half-heartedly attending a conference (though I could tell you... very little about the talks I've attended so far), which is fine except that the way this works is a screen gets shared for the duration of a presentation, then stops being shared for questions, then resumes being shared for the next talk whereupon the thing auto-fullscreens itself again and I haven't thus far even managed to find the hotkey combo that makes it Not (Esc doesn't, F11 doesn't, I was equally half-heartedly working on thesis so I haven't checked beyond that).

Is there magic? somewhere? that I'm just Missing?