I also enabled opaque window resizing in WindowMaker and played with PRICE windows containing images as well as GNUMail mail windows.
I even used one of the OresmeKit examples which has custom Views drawing data and they resize fine.
Window Maker works fine. But try Freds resize.app (posted two weeks or so ago) under unity. This will still show the gui/back bug. :-(
Unfortunately I have serious trouble with Window Maker on the concrete double screen machine I need to use for my project. When I start an app its window appears on the other screen I cannot see. I was not yet able to mirror the two displays to get rid of this problem, but that is BIOS/Hardware related and has nothing to do with Window Maker or GNUstep. Mirroring works under Unity (can be configured in display settings) but unity still has this resize / auto relayout bug, so I can switch tu Unity and thus am still in the dark (non-working state). :-(
Regards,
Andreas