Do you want to write emails in full-screen mode? If you use Mail full screen, you will be able to write, read and access your inbox at the same time. Also you may find bigger screens better than split screens and this may affect the way you work. However you may also want to disable this feature for various reasons.
Full screens means that any app or window (e.g., the Mail app) occupies the whole computer screen.
By default, you Mac will open a split screen view when you are in full screen. This means that, for example, when you click to compose a message, your screen will automatically split (see the images below). You can disable this. You can also change that new message window will slide up from the bottom of your screen. This short article explains how you can disable the split view mode in the macOS Mail app.
See also: Mac Mail App Opens Itself Randomly, Fix
1-Open the Mail app on your Mac by clicking the Mail icon. You can also open Spotlight and search for it.
2-In the Mail app, click, Mail and then Preferences (or you can press the Command and , buttons together)
3-Click the General tab
4- And now deselect “Prefer opening messages in split view when in full screen.” box
Now you are done. There is no OK or Save button. Just close the window. If you want to enable the Split View feature, just recheck this box.
See also: Mail app won’t open?