Mail app randomly opens in split-view full screen while using other apps [duplicate]

I have the same issue: The google calendar stuff is nonsense. The notification settings suggestion doesn't work.

I've narrowed it down to the following:

I normally hit the red cross button on the viewer window once I have finished looking at my mail, this closes open viewer windows but leaves the mail app active in the dock:

So if I have mail running, but no viewer windows open, then I get a viewer window randomly popping up to the foreground a few times an hour.

If instead of hitting the red cross button, I minimise mail using the minus button, then I don't get any viewer windows randomly opening. In this scenario, the mail viewer window is minimised either on the right hand side of the dock or into the application icon itself, depending on your settings (System Preferences > Dock > "Minimize windows into application icon").

So I have changed my habits to work around this annoying bug...

I have solved this problem by removing my Google account in Mail and making my Google Gmail address a normal (IMAP) account. Since then no more pop-ups.

1. Mail > Preferences > General, uncheck:
   "Prefer opening messages in split view when in full screen."

2. If you need real time notifications, best is to give the Mail app its own desktop in one corner. I've been doing it for several weeks and got no issues. You can also put some more background apps there: music ?

3. ⌘-W and ⌘-Q are your friends.
   Maybe try to get in the habit of closing Mail when watching a movie? It'll save you this hassle, a little battery life, and I promise, your mail will still be there when your movie is over. :)

4. Maybe try a different mail app? The thing is, Mail.app has been a really wonky program since time immemorial. Its one of a handful of apps that has never once got a full rewrite (along with the likes of Grapher and Activity Monitor). So I wouldn't hold your breath for a fix for these mysterious stirrings. So, if you're someone who really prefers a good desktop email client, you might want to look at a program like Unibox (super slick, ~$15, and/or available through Setapp). Outlook 2019 isn't too bad either, although it is still Microsoft Outlook, so there's "a lot there".