I've got a MacBook Air (early 2015) and time to time I have a problem with fullscreen mode. When controls disappear (cursor as well) video may become darker or change the saturation. I'm not sure what to do with this. I'm using Safari and that problem appears on YouTube.
If I try to watch Twitch in fullscreen it's always a painful process because that problem appears almost every time regardless of what browser I'm using.
Also some videos get angled in fullscreen mode on YouTube (usually these are about 4:3 screen aspect ratio)
Best Answer
Depending on the darkness of the room the Macbook Air is being used in, it may also change the brightness of the screen because the light sensor (the small gray dot to the left of the camera) is set by Apple to adjust the brightness of the screen depending on the darkness of the room. For example, if you are using your Mac at night with dim lights on, generally, the screen will darken. The darkness can be changed manually during this process with F1 and F2 (the brightness buttons)