@simonhf what you describe is the normal behaviour of Geany on other platforms, it is a multi-tabbed application, so a request to open a file from the command line opens a new tab and the command finishes, so not tying up a terminal until you finish editing.
Of course doing that makes just `geany` without any arguments useless, so it does the only thing such a command can usefully mean, it starts a new geany.