https://bugs.freedesktop.org/show_bug.cgi?id=97861
--- Comment #5 from Thomas Hume aszarsha@gmail.com --- Same problem here with my HDMI screen. Everything works fine (GNOME Wayland session and all) until I start an X server (not XWayland, actual X server from a "GNOME on Xorg" session) at which point the screen goes black (X server is mode-setting I'd imagine), and when it comes back on the purple line is present. Logging out of the Xorg session and back into a Wayland session does *not* revert the problem; I haven't tried the xrandx trick yet.
The screen is indeed blurry (enough to make it unbearable to read any text with subpixel positioning for more that a few minutes). It might comes from the scaling of the output to the remaining space (screen space - purple line space). Furthermore, any screenshot does not capture the line and it's only an 'on screen' thing.