I just downloaded the newest Zeus for evaluation ( version 3.96n, I'm currently a 3.95z user). When attempting to click and hold, then drag a window tab to a new location, sometimes this doesn't work.
Some of the time, it works as expected. Sometimes, nothing happens, sometimes there is an apparent random change in tab order.
I'm not sure, but I think there is an interaction when there are project and tool windows open and you either move them or move a document next to them.