[MlMt] Message pane has disappeared…

[Steven M. Bellovin]

While trying to hit cmd-alt-F, I accidentally typed some other key combination which made the message pane disappear—all I see is the list of mailboxes on the left and the messages in that mailbox on the right, going the full height of the screen. "View|Layout|Three Panes is selected; selecting something else and going back to Three doesn't help. How do I fix this?

        —Steve Bellovin, https://www.cs.columbia.edu/~smb