@JanAxelsson Interesting observation about conflicting Homebrew version of GTK - maybe there's a way to adjust the environment variables in the launcher script of Geany to prevent it from using the homebrew versions of the library.
@nimdanor Just a silly question but have you tried View->Show Tab Bar (or View->Hide Tab Bar) from the Geany menubar? (These are added automatically by macOS - not really something which is part of Geany itself). For me it shows the tab bar so it looks like in the screenshots and I can hide it the same way.