Did you buy this from a Ford dealer, other car brand dealer(GM, Toyota, etc), or a used dealer lot.
Also, how long ago?
If from a Ford dealer then go to a different dealer if it's under an extended warranty. If not under warranty but it's still under any short term warranty that came from the selling dealer then go to the GM of the dealership and explain that you expect the car to be fixed or you'll return the car to them. If it's outside of any warranty then call the main Ford customer service # and complain that the dealer is clearly doing nothing at all but letting your car sit on the lot when you drop it off to them.
If you bought it from another brand car dealer then threaten to contact the BBB and write a nice descriptive and well deserved review on Yelp. You may also have legal recourse if there's some kind of written record that they're supposed to fix the window. If you bought it from a shady used car dealer lot then this also applies.
It probably either needs a window reset or it needs a new window motor/module assembly or maybe a door jamb switch.
The window has a Short Drop Window function to clear the door frame/convertible top when you open/close the door. The window module might just be glitching out. Sometimes a simple reset is all that you need to do and the procedure is actually in your owner's manual.
I think that you close the door, put the window to the closed position, hold the switch in the close position for 2 seconds, push and hold the switch to the down position fully until the window bottoms out. Then that should fix it. If someone like a child was messing with the window switch when the door was open then that could have messed up the original calibration. Resetting tends to fix it.
When the basic reset doesn't fix this issue with the Short Drop Window function then there's a more drastic way to reset the window motor module: (232) Problem with window closer | Ford Mustang Forum (allfordmustangs.com)
If neither method work then you have a motor/module issue or a switch issue.