I have hige problems with using three screen on my laptop. I use the
internal display and a VGA and a DVI monitor on the docking station.

When I place the laptop in the docking station, nothing happens. I then
do the following:

 * Open system settings and configure displays
 * Arrange the monitors in the order DVI, VGA, internal (from left to right)
 * Activate all monitors
 * Set VGA as primary
 * Click Apply

At this point, kscreen seems to break down completely. The two external
monitors remain blank, and the screen on the internal display gets
messed up (showing tiles of the content all over the place). The desktop
becomes unusable.

Killing the session and logging back in leaves KDE in an unusable state
and the session crashes on login. I have to delete everything in
~/.local/share/kscreen to make it work again.

Once I do this and log in again, I get two of the three screens working.
I then do the following:

 * Open system settings and configure displays
 * Arrange the monitors
 * Activate the third monitor
 * Click apply

Now, it applies the settings, but chagnes the geometry a bit, so the
screens are not arranged directly beside each aother after applying, but
there is a gap between 1 and 2 and 2 and 3 overlap on the edge.

I then re-arrange the screens and click apply, which fixes the gemoetry.
In another step. I set VGA as primary. Applying this takes forever, then
the KDE bar finally moves to the VGA screen and everything is done.

What I can provide now is this in dmesg:

[14085.109351] [drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* 
uncleared fifo underrun on pipe A
[14085.109370] [drm:ironlake_irq_handler [i915]] *ERROR* CPU pipe A FIFO 
underrun
[14090.111493] kactivitymanage[24108]: segfault at 7fb50847ecd0 ip 
00007fb508426031 sp 00007ffc5e1d3b48 error 4 in 
libQt5Sql.so.5.4.2[7fb508412000+3f000]
[   86.841807] kscreen_backend[2331]: segfault at 10 ip 00007f5fea953bd0 sp 
00007ffd093cb508 error 4 in KSC_XRandR.so[7f5fea93e000+22000]
[   87.698749] ksplashqml[2280]: segfault at 8 ip 00007fe9533685a0 sp 
00007ffc6aca1e38 error 4 in libQt5Gui.so.5.4.2[7fe953256000+52c000]
[   87.701634] kactivitymanage[2324]: segfault at 18 ip 00007f4538f7e910 sp 
00007ffdf990aec8 error 4 in libqxcb.so[7f4538f4f000+ba000]
[   87.728423] kscreen_backend[2340]: segfault at 18 ip 00007f5df5a80b29 sp 
00007fff090273a0 error 4 in libqxcb.so[7f5df5a36000+ba000]
[   87.860003] kaccess[2343]: segfault at 18 ip 00007fb2b9cb2b29 sp 
00007ffced3b94d0 error 4 in libqxcb.so[7fb2b9c68000+ba000]
[   88.159947] ksmserver[2352]: segfault at 18 ip 00007f1551262b29 sp 
00007ffece1b7a20 error 4 in libqxcb.so[7f1551218000+ba000]
[   92.911895] [drm:intel_set_cpu_fifo_underrun_reporting [i915]] *ERROR* 
uncleared fifo underrun on pipe A
[   92.911963] [drm:ironlake_irq_handler [i915]] *ERROR* CPU pipe A FIFO 
underrun

This looks like something seriously drowns ;). Also, it remotely suggests that 
the Intel DRM component might be involved.

Reproducible: Always