Overthought

It’s a clever trick, first turning down the native backlight and then overlaying a semitransparent veil. The problem is it means the point where that break happens has to be hard coded, and since no program can adjust the backlight of a non-Apple external display, the dimming effect will always be incongruous between your internal and external displays. Also, as others have noted, a bug: I have two external displays, but this only dims one of them.

antititus about Brightness Slider