Thanks very much
for your unusually clear instructions and explanations. I really, really appreciate that you took the time to figure all of that out and tell me. I've seen bits and pieces of your suggestions in other threads, but I've never seen anything as complete and comprehensive as what you wrote here. I've bookmarked it and also printed it out to keep in my "Linux Notebook."
Unfortunately, even after doing everything that you suggested, nothing has changed.
I kept all of the changes that you suggested, except that I re-enabled the framebuffer, since it didn't make any difference to comment it out.
1. 3D acceleration was enabled before -- it's still enabled now.
2. glxgears -printfps worked before -- it still works now, and it reports the exact same frames per second that it did before.
3. Of the 76 OpenGL screensavers that appear as choices, only the very few that worked before are working now.
Does 76 OpenGL screensavers seem like an awful lot of screensavers to have been installed by default by the Debian KDE installation? I would've been surprised to find 76 screensavers all together, let alone 76 OpenGL screensavers, PLUS all of the non-OpenGL screensavers.
I wonder if maybe I was supposed to "get" only the OpenGL screensavers that are currently working, but the installer somehow incorrectly added the names of all of those other OpenGL screensavers to my list of choices.
At least at this point, I KNOW for sure that OpenGL itself is working correctly, even though only a few of my 76 OpenGL screensavers are working.
BTW, I was very careful to DE-select DRI during the dpkg-reconfigure xserver-xorg portion of installing the nvidia drivers "the Debian way." Apparently, from what I can tell with my limited knowledge, it looks to me like de-selecting DRI during that process doesn't REMOVE DRI from xorg.conf -- instead it leaves the DRI lines in there but it doesn't put in a line that will load DRI. (???)