The Disappearing Mailboxes
The native macOS Mail application uses a three-pane layout: a sidebar on the left showing your mailboxes (Inbox, Sent, Trash), a middle column listing individual emails, and a large preview pane on the right. However, if you attempt to focus entirely on your email by clicking the green button in the top-left corner to enter “Full Screen” mode, macOS Mail implements a highly controversial design choice. It automatically collapses and completely hides the left sidebar, forcing you to move your mouse to the extreme left edge of the screen every single time you want to switch from your Inbox to your Sent folder.
Pinning the Sidebar
To maintain your workflow and stop macOS from automatically hiding the Mail sidebar when entering full-screen mode, you must toggle a specific view setting while in that mode.
First, open the Mail app. Click the green traffic-light button in the top-left corner to enter Full Screen mode. You will notice the sidebar immediately vanishes. Now, navigate your mouse pointer all the way to the absolute top edge of the screen to reveal the hidden macOS Menu Bar. Click on View. In the drop-down menu, look for the option labelled Always Show Sidebar in Full Screen. Click it so a checkmark appears next to it. Instantly, your mailboxes will permanently slide back into view, locking the standard three-pane layout in place, no matter how you resize the window.