By default, when the computer is inactive for 120 seconds the screen brightness is automatically reduced. This is a great idea, except that the brightness is also reduced when playing a video file with VLC, suddenly everything gets darker and is annoying to move the mouse every 2 minutes to restore the brightness level.
So to fix this all we have to do is get rid of this option. Open the Power Manager Settings and then on the "On AC" and "On Battery" sections select the Monitor tab. Adjust the third slider all the way to the left, as shown in the picture bellow.