Apologies, I’m mixing up things. Fixed functions are on hardware. It’s just configuration/state-machine based as opposed to programmable with shaders. And compatibility profile is indeed core + fixed functions, sorry.
Without drivers, Windows defaults to software rendering on the CPU and Mesa can fall back to software rendering as a last resort, but these are different from OpenGL fixed functions.
Regarding the issue in this thread, I think the investigation done by @lassoan and the Slicer team is the most exhaustive so far.
Agreed, thanks for redirecting me to this. Apologies for cluttering this discussion with erroneous comments.