To me this looks like a duplicate of #1729 - the problems are caused by the extra tab at the top which shouldn't be there. To summarise what you could try to fix the problem:
1. In macOS Settings set the value of "Dock->Prefer tabs when opening documents" to Manually. 2. In Geany menu bar try selecting "View->Hide tab bar" 3. As suggested in #1729 you might want to remove a GTK installation from homebrew or some other package manager which might conflict with Geany.
I'm closing this issue as I think it's a duplicate of #1729 and the discussion can continue there. Feel free to reopen this bug report if you think it's not the case.