First a comment, with Linux Mint Cinnamon, default settings, open-in-geany in the file manager (Nemo) always brings Geany to the front with the notebook page containing the selected file having focus, both when Geany is first started, and when the selected file is added to an open instance.
Everybody who is complaining, the problem is not with Geany, it is with your window manager and/or the settings you use that tell the window manager not to grant Geany's focus request. Perhaps enquire on your window manager support what settings need to be changed, or if you need to use a different window manager.