On OS X, when game enters fullscreen, after a few seconds, or intermittently,
game gets dark suddenly, and remains dark until game is exitted.
While dark, if you go to Brightness control and touch the slider, it instantly reverts to correct brightness again... for a while anyway.
Damn annoying thing no-one seems to know much about.
Building SDL and quakespasm the traditional unix way (libSDL.so) seems to get around it.
Is a major reason why I ditched hardware gamma in Mark V. It stupidly affects Windows and OS X. I wrote tons of code trying to catch the system event that mucks up the brightness in Windows after switching between fullscreen and windowed mode. There isn't one. Thread has some interesting infos on the topic: http://forums.inside3d.com/viewtopic.php?f=3&t=5185