--- Comment #73 from Dik Takken <> ---
I may have figured out what is causing this issue. I ran

xrandr --verbose

a few times and diffed the output between reboots. As it turns out, the
reported CRTC for the same external monitor changes randomly between reboots.
Disabling and re-enabling the display restores the CRTC reported by xrandr. 
This is what I see happening:

I start out with a correctly operating dual-screen setup, two identical
displays attached to the dock of my laptop, desktop extends across both
After reboot, I often see the following things happening:

1. login screen and KDE splash screen extends across both screens, as it
should. Desktop appears, one of the screens is black. I can move the mouse to
the black screen, but no response to mouse clicks.
2. Open display config. Disable secondary monitor. While doing this, the
display resolution for the now disabled screen is reduced.
3. Enable the display again. Desktop extends again to both displays
4. Restore resolution of the just enabled display

I captured the output of xrandr, kscreen, and the changes in the kscreen
display configuration file after each of the above steps 1 to 4, see
attachments. You can see how the  CRTC changes after disabling and re-enabling
the secondary display.

Hopefully, this will pin down what the source of the problem is.

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