Mail opens a window randomly in split screen with my fullscreen window
I've been having the exact same problem for a while - particularly interesting while I'm trying to watch netflix!

I found a checkbox option in the Mail->Preferences tab which says "Prefer opening messages in split view when in full screen" (see screenshot). I've unchecked that so here's hoping it solves the problem!

If it doesn't, I've seen a few other posts about this suggesting that the fact that it pops up at all is something to do with Google Calendar if you have any Google accounts assigned to your mailboxes. See if this link helps you if so!

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