Since you can run several instances of Geany with or without any open project at the same time, drag a document tab from one instance and drop to other could be useful. Also if you press Control during the action the tab is duplicated.
The feature request is five years old already, so is anyone working on this?
I really need drag 'n' drop of tabs, because I always work with two Geany instances.
If you developers do not have time at all for this, how hard would it be to implement for someone who hasn't contributed to the geany code yet? I might give it a try myself then, if I have the time.