hanging unloaded doors?

It sounds crazy but there are legitimate reasons to hang and align empty door shells - before and after paint - including compensation for weight of the glass/hardware. There's a lot of extra weight with all the hardware and even with fresh hinges they will want to droop at the rear - albeit very slightly.

Examples: Empty shells are much easier to work with during a build mockup. I had to add material to a donor door edge in one spot to correct a gap issue where it meets the rear quarter and jamb that were also replaced. My car was also painted in pieces. The doors were later hung (empty) and the hardware re-installed - much easier than hanging and adjusting a fully loaded door. You may still have to make minor adjustment. With that being said, I didn't have to deal with quarter windows so your experience may be vastly different.