Reader Darryl Rehr sent in a tip about a new keyboard shortcut in macOS that baffled not just him by its previously unseen behavior, but also a ton of Apple phone support staff. “Near the end, they nearly had me erase my hard drive and restore from Time Machine. Thankfully, I was skeptical,” he writes.
Darryl found all his messages in Mail missing except new ones. He assumed corruption or other problems, and went through troubleshooting and then calling Apple support. The culprit was Command-L, which triggers filters. This keystroke isn’t used for this purpose in previous releases.

The filter menu provides a couple of visual cues that filtering is active in the modern Mail view.
If you’re using the modern Mail layout, default for multiple releases, there’s a visual indicator at the top of the messages list that has a blue label showing the applied filter, like Unread, and the filter icon reverses out to mostly black. It’s still a bit subtle, but it’s at least noticeable. (It doesn’t seem that you can define more of these filters; Mail comes with several prefab ones.)
However, if you’re using the classic layout for Mail, as Darryl was, there’s no onscreen indicator at all! You’d have to know you pressed Command-L by accident, or know to look in the View menu, where an item labeled Disable Message Filter appears. (You can turn on the classic look in Mail > Preferences > Viewing, where you check the box Use Classic Layout.)
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