Watching a video via newpipe, I paused the video and Something Happened
The brightness went down for fullscreen and stayed down, no matter what I did. I couldn't figure out how I had done it and how to reverse it. Closing and restarting the application didn't help either
Turns out you have to go to the settings and set "left screen gesture" from "brightness" to "none"
Gestures are a terrible way to control an application, even on a smartphone, and especially if they're completely undocumented and random like this
If you are a developer who likes implementing non-features like this: don't