Bug#846877: plasma-desktop: Sddm can not start kde. Then panel is not working.

Hi,

I had a similar issue. The symptom is plasma showed me a desktop without widgets and panels.
After some investigation, I found it might be due to dual monitor support.

Below error is from boot log.

[drm:intel_dp_link_training_clock_recovery [i915] *ERROR* too many full retries, give up
[drm:intel_dp_start_link_train [i915]] *ERROR* failed to train DP, aborting

I had my normal desktop back after I disabled the 'fake' laptop monitor, selected the real monitor as default in system settings.

My main board has a HDMI port and a VGA port. My monitor is connected to VGA port.
Due to some unknown issue, Debian thought there was a laptop monitor in HDMI port.
Plasma took the fake laptop monitor as the default monitor and the real monitor as secondary monitor.
So the desktop was double the width of my real monitor.
The right half of the desktop was shown in my real monitor; the left half, with panels and widgets, was shown in the fake laptop monitor thus invisible.

Workaround Steps:
1. Right click in the desktop screen, click 'Run command'.
2. Type systemsettings5 in the input box, hit ENTER.
3. Click Display and Monitor under Hardware category.
4. Click Displays on the left side.
5. Click the laptop screen, uncheck Enable box, click Apply.

If systemsettings5 was started in the fake monitor, you need to restart sddm, then use systemsettings5 -geometry +XOFF where XOFF is the width of your screen in pixel.

This seems a bug in display driver, not in plasma-desktop.

Hope this is helpful.