Mac Mail: Stuck separator bar
The separator bar is stuck at maximum, so there's no way to view an email without double-clicking and opening it individually. On hover, the bar's cursor changes shape as if to adjust its location, but won't budge off the maximum position. I have tried restarting the program and the computer several times. The problem sticks whether I am in Classic view or the mail-list-on-the-side view.
Mac mini, macOS 10.13