Wow, that's crazy, it actually works -- just tried out the sample videos in QuickTime on my 2016 13" MBP (P3 gamut, running Big Sur) and confirmed working.
Basically: if I set my display to ~75% brightness and open the video, the whites in the video are 100% brightness, way brighter than #FFF interface white on the rest of my screen.
But if I increase my display brightness to 100%, the whites in the video are the same as the interface white, because it obviously can't go any brighter.
If I decrease my display brightness to 50%, the whites in the video are no longer at maximum 100% brightness, maybe more like 75%.
But it's also kind of buggy -- after messing around with system brightness a bit, the video stops being brighter and I've got to quit QuickTime and restart it again for effect. Also, opening and closing the video file makes my cursor disappear for a couple seconds!
I'm wondering if it switches from hardware dimming to software dimming when the video is opened and closed, and if that switch has to do with the cursor disappears. If it is, though it's flawlessly undetectable in terms of brightness -- the interface white and grays don't change at all.
Interestingly confirming it: taking a screenshot of the video while screen is at 75% brightness show massive brightness clipping in the video, since it's "overexposed" in the interface's color range. But taking a screenshot while screen brightness is 100% shows no clipping, because the video is no longer "overexposed" to the interface.
I'm just so surprised I had utterly no idea macOS worked like this. I'd never heard of this feature until now.
Basically: if I set my display to ~75% brightness and open the video, the whites in the video are 100% brightness, way brighter than #FFF interface white on the rest of my screen.
But if I increase my display brightness to 100%, the whites in the video are the same as the interface white, because it obviously can't go any brighter.
If I decrease my display brightness to 50%, the whites in the video are no longer at maximum 100% brightness, maybe more like 75%.
But it's also kind of buggy -- after messing around with system brightness a bit, the video stops being brighter and I've got to quit QuickTime and restart it again for effect. Also, opening and closing the video file makes my cursor disappear for a couple seconds!
I'm wondering if it switches from hardware dimming to software dimming when the video is opened and closed, and if that switch has to do with the cursor disappears. If it is, though it's flawlessly undetectable in terms of brightness -- the interface white and grays don't change at all.
Interestingly confirming it: taking a screenshot of the video while screen is at 75% brightness show massive brightness clipping in the video, since it's "overexposed" in the interface's color range. But taking a screenshot while screen brightness is 100% shows no clipping, because the video is no longer "overexposed" to the interface.
I'm just so surprised I had utterly no idea macOS worked like this. I'd never heard of this feature until now.