A full-width line appeared at the very top of my laptop screen. It shifted color with the background but never disappeared: dimmest on white, brightest with a green/red fringe on black, blue and red. That rules out content bleed and points at hardware, a bad gate line or a loose eDP cable. Overlay masking is useless for this: painting black there makes it more visible, not less.
Workaround below shifts the whole image down past the bad rows and keeps the mouse cursor out of the resulting dead strip. Tested on Linux Mint / Cinnamon, X11, 1920x1080 internal