Can't move the separator between the message list pane and message pane.

I have two TB instances; one for personal mail and one for my work. A few weeks ago I upgraded TB to 60.3.0. Since then, I was unable to move the separator bar between the message list pane and the message pane in the personal instance (both instances use the "classic" layout, with a folder list on the left and the message list and message on the right, with the message pane below.) The two panes were "stuck" in a position showing only three messages in the list. I've lived with that, but it's been a bit of a pain. (Note: I don't really know whether the upgrade caused the problem.)

Today, I upgraded to 60.4.0, hoping it might solve the problem. It didn't, but I got a "good enough" solution by changing to the vertical view, and then immediately back to classic. Now, when the mouse hovers over the separator bar, it changes to a double ended horizontal arrow. Ignoring the orientation of the arrow, I can move the bar up and down, which is what I wanted. (I say "good enough"; "perfect" would be that the arrow would be vertical.)