In KDE Plasma 5, I started a new session and while the session was not completely loaded, I start to tab into another activity, but got a completely black screen, with only the white mouse pointer that I still could move on it.
I went into a TTY console and did:
ps aux | grep X
and kill the pid of the X process.
I was immediately brought back to the session login screen (sddm).
I logged in again and I saw an X window with the mention:
"Could not start ksmserver. Check your installation".
