Please note: I'm not sure whether this bug is filed under the correct product.

SUMMARY
Disconnecting an external monitor from a *closed-lid* laptop and _trying_ to continue work on the laptop causes the laptop screen to be black with the mouse pointer. CTRL+ALT+F{2...9} + CTRL+ALT+F1 doesn't help. Only way to get the laptop display working is to either re-connect the external monitor or reboot the laptop.

There is NO problem if the laptop lid is open when the disconnection happens which makes me think whether this could have something to do with sleep – when the external monitor is disconnected the laptop goes to sleep, of course, since the lid is closed. Or maybe it's not sleep but it's the lockscreen which is a result of waking from sleep. No idea.

STEPS TO REPRODUCE
1. Use an external monitor, laptop lid closed
2. Disconnect the external monitor
3. Try to continue working on the laptop

OBSERVED RESULT
Screen is totally black with mouse pointer which can be moved

EXPECTED RESULT
Screen is normal and shows all the usual stuff

SOFTWARE/OS VERSIONS
Thinkpad T480s
Operating System: Kubuntu 19.10
KDE Plasma Version: 5.17.3
KDE Frameworks Version: 5.64.0
Qt Version: 5.12.4
Kernel Version: 5.3.12-050312-generic
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-8650U CPU @ 1.90GHz
Memory: 23,0 GiB of RAM


ADDITIONAL INFORMATION