This is in Menu -> System -> Hardware Drivers and has an icon resembling a PCI card. I tried it and it said no proprietary drivers are in use but they are needed for 3D graphics. This is in spite of having run EnvyNG and "nVidia X Server Settings" reporting that drivers 173.14.12 are installed and in use. So out of curiosity I enabled the Hardware Drivers check box and rebooted.
Two hours later and finally remembering that Envy is now called EnvyNG, and then reinstalling EnvyNG's drivers, everything is back to normal.
But what happened? And, perhaps more importantly, what is that Hardware Drivers app doing there if it incorrectly reports that the wroing drivers are in use? If anyone can duplicate all this (thereby showing that it's not some hardware issue pertaining just to me), then that app should be removed before M5 KDE-Final is released.