Created attachment 212861 [details]
faulty image after running xrandr

This is a Dell XPS 13 9343 (2015) with a native resolution of 1920x1080. If I boot it with an external screen (3440x1440) attached via DP, the image from the laptop gets mirrored automatically when X starts. If I then attempt to change the resolution and layout by running

xrandr --output DP1 --auto --right-of eDP1
  or
xrandr --output DP1 --mode 3440x1440 --right-of eDP1

then X becomes unresponsive 4 out of 5 times and I have to kill it from another VT (e.g. TTY2). The resolution of the external screen does indeed change, but the image from the laptop is tiled a bunch of times and I can't interact with any windows.

Attached is a screenshot I managed to grab by running

xrandr --output eDP1 --auto --output DP1 --auto --right-of eDP1 & (sleep 4; scrot)

In the screenshot, the upper left tile is on my laptop as I expect it. The part on the right is supposed to be the DP screen with the unexpected tiling.

Xorg.0.log is attached. The output regarding xrandr (timestamp 3041.823) seems normal.

I've had this problem in both linux 4.4 and 4.5 and I think it started with xf86-video-intel 1:2.99.917+587+gc186d4d-1 but it's hard for me to say, exactly. I marked it as a regression because I didn't have this problem for several months prior.

More info:

> xrandr # output when it's working correctly and as expected
Screen 0: minimum 8 x 8, current 5360 x 1440, maximum 32767 x 32767
eDP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 294mm x 165mm
   1920x1080     59.93*+
   1400x1050     59.98  
   1600x900      60.00  
   1280x1024     60.02  
   1280x960      60.00  
   1368x768      60.00  
   1280x720      60.00  
   1024x768      60.00  
   1024x576      60.00  
   960x540       60.00  
   800x600       60.32    56.25  
   864x486       60.00  
   640x480       59.94  
   720x405       60.00  
   640x360       60.00  
DP1 connected 3440x1440+1920+0 (normal left inverted right x axis y axis) 800mm x 335mm
   3440x1440     59.97*+  49.99    29.99  
   2560x1080     60.00  
   1920x1080     60.00    59.94  
   1680x1050     59.95  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1152x864      59.97  
   1280x720      60.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
HDMI1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis

> pacman -Qi xorg-server xf86-video-intel | grep -E "^(Name|Version)"
Name            : xorg-server
Version         : 1.18.3-1
Name            : xf86-video-intel
Version         : 1:2.99.917+622+gde44aaa-1