The other day my email list suddenly displayed in Compact mode and I didn’t understand why. I checked the UI for a button I may have clicked, and the Shortcuts for a key I may have pressed, but nothing. It is only after digging through the documentation I found the email list was turning to Compact mode automatically when above a certain width, which is configurable, and the behavior may also be disabled in Preferences/Email.
I don’t find this default behavior user friendly at all, I would suggest to turn off the automatic switch to Compact mode by default. Also I would suggest to add a button in the UI to turn on/off Compact mode.