Created attachment 116639 [details]
Xorg.log showing the various state changes before getting the display mirroring going.

When using an external display on a HP ProBook 640 G1, the external works only partially. The external display can reliably only be used as a mirrored display. Switcing to any other mode is a hit and miss. Also disconnecting or connecting external displays when X is running causes X to crash.

How to reproduce:

Example case 1:
- Connect an external display to the Docking station through DP port
- Start the computer
- Try to switch display mode from the keyboard (fn+F4)
- Observe the display state changes, the external display works only in a mirrored mode

Example case 2:
- Start the computer (and allow it to boot to X)
- Connect an external display
- X Crashes

In the logs I used a Docking station to reproduce the problem, but the same issue presents itself on the built in connectors as well.

Guesswork:
The reason why I reported this as a single issue and not 2 separate issues, is that it looks like the Display identifiers are crossed somewhere. If I try to turn off the internal display with xrandr, the external display will turn off etc. This would cause the X server to get confused when a new display is connected and cause it to crash. But this is just guesswork on my behalf.


The issue was reproduced with the following environment:

chipset: Intel HD Graphics 4600 (Integrated in the CPU I5-4200M)
system architecture: x86_64
xf86-video-intel/xserver/ version: 2:2.99.917-1~exp1ubuntu2build1 (From Ubuntu 15.04)
kernel version: 3.19.0-21-generic (From Ubuntu 15.04)
Linux distribution: Ubuntu 15.04
Machine or mobo model: HP ProBook 640 G1
Display connector: eDP and DP