Sometimes, on my Arch Linux laptop, using GNOME for the desktop, my screen will fade out after a while of non-activity (even if I'm watching a video). This fade out is very slow, and can't be cancelled by mouse movements, keyboard presses etc. What is responsible for this, and how do I disable it?
The fade out is probably the screensaver kicking in. Try to disable it by going to System->Preferences->Look and Feel->Screensaver and disabling "Activate screensaver when computer is idle" if indeed the active screen saver is "Blank screen".
The fact that the fading out can't be interrupted is a bug it seems. E.g. Fedora has a bugreport stating it is a fault of the X server and fixed with an update.