(In reply to Diego Viola from comment #65)
> If I set that environment variable while I'm already on X:

By "X" I assume you mean Xorg here?

> export MESA_EXTENSION_OVERRIDE="-GL_NV_texture_barrier"
> 
> It has no effect on Xephyr (from current git master), i.e. xterm still shows
> the bug.

Did you set the variable before starting Xephyr though, so it was set in the
Xephyr process? If yes, that makes things confusing again, just when it seemed
like they were starting to make sense...