A fresh user-account runs Geany fine. On the same mac as the bad account
On the bad account, I have tried to look in $HOME/.config and removed everything related to geany and gtk, but it does not help. Still opens geany with OSX's own title bar, and the GTK titlebar inside the window (as in figure above in this thread).
This means that there is something very account-specific which I can't isolate.
I know this may not be Geany's fault, but it is still frustrating, and I would really appreciate if somebody gave me some hints. If the problem is understood, I do trust that this could be used towards Geany becoming robust against such problems in the future