While I hate to be "that guy" who posts an answer to his own question, I went through more than a little pain to get things working, and I'd like to save the next guy the same amount of trouble. So, here is
What actually ended up working
For reasons I do not understand, my system worked when I ran contrary to the (quite sensible) recommendation from @allquixotic.
1) Disable LIBGL_ALWAYS_INDIRECT
This wound up being really difficult, simply because I had to find where it was set.
- In the folder
\etc\profile.d was a file wsl-integration.sh.
- The above file was actually a symlink; the actual file was
/usr/share/wslu/wsl-integration.sh
- In that file the variable
LIBGL_ALWAYS_INDIRECT was set, so I commented out that line.
2) Do not use the -wgl command line argument (or its GUI equivalent) for VcXsrv
Since I was launching VcXsrv from a GUI client, this meant leaving the second option box, titles "Native opengl", unchecked.
Only once I had both those changes made (and did a fresh reboot to make sure no old settings for VcXsrv had persisted) then the gears in glxgears were turning at a normal rate, and I could reorient them using the arrow keys, just like they're supposed to work.
Best Answer
You might want to try this benchmark. There are a few more on the site if this is not what you are looking for (downloadable by mirror torrent interestingly enough).