I have a HP EliteBook 840 G2 running openSUSE Leap 15.0 with all patches
applied.
The laptop runs fine when it is in normal portable mode (off dock), also with
an external monitor. When I dock it into its docking station at home, I
experience weird graphic device behaviour. It is a matter of luck if it is able
to initialize and configure the attached external monitor the way it is
expected. Sometimes it hangs in an endless config loop when trying to
initialize the monitor.
The external monitor is a SAMSUNG U28D590D connected via display port to port
#1 to the docking station.
However, if I undock the laptop and plug the very same display cable to the
onboard display port then the display and the laptop run as expected. It really
is an issue I can only experience when putting the device into the docking
station.
For comparison: I had the chance to try another device, a HP EliteBook 840 G3
with the very same docking station and monitor. It was running Windows 7 and I
did not see any strange behaviour. Additionally, I was earlier running openSUSE
42.3 on the very same device configuration and did not have any issues. The odd
behaviour only started after upgrading to Leap 15.0 and got worse with each
patch cycle, to reach total unusability as of today. Hence, thats why I am
writing this bug report.
One thing I occasionally see is this error message in the logs:
kernel: [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe C
FIFO underrun

I attaching output from dmesg, journalctl, and xrandr in different
configurations. Let me know if you need more.
Thanks for the support.