The bottom part of my laptop screen is completely black after I log in (but not before; I'm using SDDM as my log-in manager with plasma 5.6.4, Qt 5.6.0, Frameworks 5.21, kernel 4.5.4-1 ARCH). About an inch off the bottom of my screen is completely blacked out.

1. The system knows there is an area there. Whenever I maximize a window, it uses that portion of the screen, but I cannot see anything since it's just a black rectangle.

2. My screen also happens to be a touchscreen, and when I touch that area, it takes me to the corresponding area that is not blacked out.

3. When I connect an external monitor (using HDMI), the entire laptop screen becomes usable again and the black part is filled with the rest of the screen.

4. I don't know if this is related, but this coincided with me being unable to adjust the brightness (I could do it before).

Does anyone have an idea how to debug this? Thanks!

i have something similar.
Plasma 5 on Mageia 6 sta1
It was fine until couple of days ago.
The machine only reinstalled as an HTPC a week ago needed a reboot to restore lost wifi.
That worked but a rectangle occupying about 2/3 of the screen from top left.
Programs can be slid from beneath it, with difficulty.

The rectangle forms from the black KDE start up screen with its cogwheel-K logo freezing and then shrinking to this rectangle.

No solution so far.