Sometimes when I dock the notebook to a docking station with two external monitors attached the desktop of the two external monitors turns black and the plasmoid disappear, while the mouse is visible. (If I'm not wrong also the other activities disappear)
Then going to System Settings > Power Managment and clicking on the activity tab the desktops reappear as they were.
It seems that when contacting the Activity dataengine (maybe) something is triggered and everything goes back to normal state.
Steps to Reproduce:
1.Dock the notebook to a docking station
Black desktop on external screens and no plasmoids
Screens turn on showing normal desktops
Dell E7440 notebook connected to a docking station where two other monitors are connected
May be related: https://bugs.kde.org/show_bug.cgi?id=345225
Sounds to me like Plasma not correctly detecting screen changes (i.e. we turn the screens on, but Plasma won't create background and panels on those screens).
Can you test this with Plasma 5?
I'll see if I can try a live plasma 5 image to test it in the next days.
Simone, any update?
I've tried briefly with an image of Plasma 5 and it was not happening, but it's not a conclusive result since the problem appear rarely and I didn't find a pattern to reproduce it yet. I'll update to Plasma 5 soon, so I can carry out more extensive testing.
Is this similar to https://bugs.kde.org/show_bug.cgi?id=351025 or https://bugs.kde.org/show_bug.cgi?id=352186
I'm using Plasma 5 since few versions now, and even tough the multimonitor support is still completely broken both from KScreen and Plasma side unfortunately, this specific problem haven't happened anymore.
This problem appeared again in a different form. Sometimes after boot, the external monitor is black without plasmoids, without the right click menu, but the mouse cursor is visible. Moreover the panel that should be in this screen is in the laptop screen.
Restarting plasma solves the problem.
It seems that the external monitor is not detected by plasma and kscreen.
Didn't happen in a long time. I mark it as solved.