Before anyone gets too excited, it's not the "Adaptive Brightness" feature of the OS. I've already turned that off.
Also it seems to have nothing to do with ambient light. It actually seems to do with the average "color" of the display. If I'm working in dark-themed Visual Studio, the brightness "pops" brighter. When I switch to the browser, it "pops" darker. So it's kind of adaptive brightness based on average pixel color (or something like that).
What makes it rather annoying is that the brightness pops, rather than transitioning gradually. What is this feature, and how do I disable it (or at least make it smoother)?