Since Catalina our movies keep getting interrupted by the Apple mail app randomly opening. One minute we are watching a movie then next moment half our scene vanishes and the Apple Mail appears! Thanks to my son Tim who worked out the solution below.
If you close Apple mail (Apple-W or hit the red circle) then if Apple mail has problems making a connection it will pop up fullscreen next to your movie. If however you hide mail, (Apple-H) then it won’t pop up and interrupt your movie.
It turns out to be some kind of a problem with Apple’s mail app that when it can’t connect to your server it comes up full-screen. My theory is that when you are watching a movie perhaps at times this makes it hard for mail to check the mail. Hiding mail instead of closing the window is a good workaround.