Freeze center divider in vertical dual mode

I'm using dual vertical mode with folder trees in both panes.

Somehow I've set it so that when I adjust the tree/file divider in the left-hand lister pane, it drags the divider between the two lister panes along with it. (Doesn't happen on the right.)

How can I get back to a central lister-pane divider that stays put in the center while I adjust either side?

Double clicking the divider(s) may reset them, if it was working how you wanted initially.

Yes, but I have an unfortunate tendency to fiddle with the tree/file display dividers. And each time I do that -- on the left side only -- the center divider between the dual lister panes is sort of randomly adjusting, left-right. As though it was trying to auto-adjust for columns.

No way to freeze that central divider in a fixed position?

Thing is, this behavior is new: I used to be able to adjust each lister pane separately -- adjusting one didn't push and pull the other out of place.