Did your Mail sidebar disappear in macOS Tahoe? Here's the fix
If you opened Apple Mail one day and found that all your folders and inboxes had vanished, you probably weren't imagining things and you didn't accidentally delete anything. macOS Tahoe added a small new button to Mail's toolbar that toggles the sidebar on and off, and it's very easy to click without meaning to.
Hovering over the sidebar button, a tooltip pops up to show you what the button does.
What is the sidebar toggle button?
The button looks like a small rectangle with a vertical panel on the left side, the standard icon for showing and hiding a sidebar. Apple added it in Tahoe to give you a quicker way to collapse the sidebar and get more reading space. On paper, that's a useful feature. In practice, several of my clients have accidentally brushed it while clicking nearby buttons and then spent a frustrating few minutes wondering where their inbox went.
How do you get the sidebar back?
Click the button again. That's all there is to it. The same button that hid the sidebar will bring it back. It's in the upper-left area of the Mail toolbar, to the left of the search field. If you hover over it, a tooltip should confirm it says "Show Sidebar" or "Hide Sidebar."
The folders are missing (but not gone!)
How do you remove the button if you don't want it?
If you'd rather not have this button in your toolbar at all, you can remove it:
Control-click (or right-click) anywhere on the Mail toolbar
Choose Customize Toolbar...
Drag the sidebar button off the toolbar and release it
Click Done
The sidebar will still work (you can still show and hide it from the View menu), but the accidental-click problem goes away entirely.
Right click (or control-click) the bar and choose “Customize Toolbar”
Simply drag the icon you don’t want off the bar, then click the blue “Done” button.
This is one of those small changes Apple makes that rarely gets announced anywhere, so when something stops working, it can feel like your computer is broken. It isn't. If you run into other unexplained Mail weirdness after updating to macOS Tahoe, I offer one-on-one tech tutoring sessions in San Francisco and Washington DC, and via Zoom, where we can work through it together.
