The text window in the map view is either
all the way closed or takes up half the screen
Used to be able to have it any size.
Help
Please raise this on an existing thread on this problem: Divider line "snaps back"
This is not criticism of the post, but simply avoiding parallel threads on the same issue. Nor is it critique for not having spotted the connection—which is easier for admin/mods to spot.
This is a known issue, q.v. linked thread, but—importantly—as yet undiagnosed issue. A fix depends on the ability to reproduce the issue on demand and on more than one Mac †.
†. You’d be surprised as how often odd things can occurs (with a large tool box) where we can’r reproduce the problem affecting one person (I’ve been one such). Annoying if it’s you but I’ve come to see too that if it can’t be reproduced it’s hard to know what is being fixed. 
Close Tinderbox and reopen; it will return to normal.