I installed the gnome-panel utility, but when I switched to Gnome Metacity, windows there appeared to have no shadows. Here's a picture:
It's actually very difficult to work against white backgrounds with windows that don't have clearly delineated borders. I'm wondering if there is a way to somehow get those drop shadows back?
It seems to be only a problem in the new Ubuntu 15.10. I don't think there's any simple way to fix it. So, I just reverted my computer back to Ubuntu 14.04.