I'm using a laptop alternating two-monitors setup to single monitor arrangement when moving. 
Description of issue:
starting from a working one-screen setup, I turn off the laptop
then I connect the external monitor and turn on the laptop
The result is that the applets are on the laptop screen and the same screen is ok and it shows the wallpaper. External monitor is another story: no wallpaper (black background), no panels and right mouse button does not shows any context menu.
I can , however, drag windows on external monitor with no problems.
If I disconnect the external monitor and then plug-in it again, the change is noticed by plasma, wallpaper appears on the external monitor and right mouse button works again. I can also drag panels to the external monitor.
If I then turn off the laptop, without disconnecting the external monitor, I'm back to square one: turning laptop on I get again no wallpaper and other functionalities on external monitor, as described at first step above.


Reproducible: Always




plasma 5.6.5 (plasma-workspace-5.6.5.1)
kde-frameworks 5.23.0
kde-apps 16.04.2
Gentoo packages