Taking Mail full screen can be a good way to focus and crank through your inbox. Fortunately, you can set it to open messages and new drafts in an adjacent Split View, which keeps your message list, a separate viewer, and your mailboxes still accessible. The message side of Split View will even open multiple messages and drafts in tabs.
In the Mail app on your Mac, move the pointer to the green button in the top-left corner of the window, then choose Enter Full Screen from the menu that appears or click the button. With Mail full screen, do any of the following: Open a message: Double-click a message in the message list. Each message you double-click opens in a new tab. Mail Preferences General, uncheck: 'Prefer opening messages in split view when in full screen.' 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. I just tried unticking the box 'Prefer to open Mails in split view when using full screen' on the General tab in settings (⌘,). It might be a bug in that script, but I'm not sure if it helps yet. Mail App opens randomly in split screen Saw a post about this about a year ago with no solution to the problem. It's so frustrating, it opens by itself daily when I'm using the browser without me touching it. No, it's not that. Mail app is not open - it's just running in the background. Periodically, when I'm in full screen (working in another app), Mail just opens up by itself and takes up the right side of the screen - just like SpiritSoul1008 said. But I give it a try and deselect this option - who knows, maybe it helps.
Check out the gallery below to get the idea.
Mac Mail App For Windows
To enable this Split View behavior, to go Mail’s Preferences > General pane and toggle Prefer opening messages in Split View when in full screen. I know, the name doesn’t exactly roll off the keyboard. But it’s a handy option that I’d like to see in more Mac apps.