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)