Symptom discovery:
A while ago, running quake4 in fullscreen started shifting the entire display up by a few pixels. Bizarre.

Quake4 output included a message from SDL_ListModes listing 2560x1669 as the first mode, instead of the correct 2560x1600. Strange.

xwininfo reported: Size: QSize(2560x1600), but with "Corners: +0+69 -0+69 -0-0 +0-0", and Pos: QPoint(0,69)

Meanwhile, Nvidia server settings showed for X Screen 0, dimensions: 2560x1669--this would explain SDL's output--it was just a symptom.

Kde settings display gui showed correct 2560x1600 display size, no mention of a 69px or 1669 anywhere. Nothing else obviously stood out indicating why a 69px shift was occurring.

Xorg.0.log had entries:  Setting mode "DP-1: nvidia-auto-select @2560x1600 +0+69 {ViewPortIn=2560x1600, ViewPortOut=2560x1600+0+0}
This entry led me on a wild goose-chase thinking it was an nvidia/xorg/edid problem.

playing with kscreen-console I saw the "config" command, and discovered this bizarre entry with:
"pos": { "x": 0, "y": 69 }

logging out, stopping xdm, editing that 69 to 0, and restarting xdm everything is now as it used to be.




Reproducible: Didn't try

Steps to Reproduce:
I have no idea what would edit that file, or why it would get a value of 69 for that entry.
I just have the one monitor, and don't normally mess with the display settings other than to restore it after a fullscreen app (at lower res--usually a game) doesn't properly restore it upon exit.



Expected Results:  
I would expect that the display settings GUI would make it *blatantly obvious* that there is an offset being applied to the screen dimensions, and an *intuitive way* to adjust/remove it. I shouldn't have to wade through log files and rarely-used commands to discover there's a config file buried somewhere with a single bad value.

Furthermore, I don't know what I did that caused this, thus I don't know how to ensure it doesn't happen again. At least now I know where to look to fix it now--but afaik I still have to kill my session and manually edit that file to change it.

libkscreen 1.0.5
kdm 4.11.21 and 4.11.22
xrandr 1.4.3
libXrandr 1.4.2
nvidia drivers 352.21 and 355.06-r1
xorg-server 1.16.4-r2

This problem persisted across an update from 4.11.21 to 4.11.22 and nvidia driver update--leading me to believe it was just that bad entry in that file causing it all.