More than occasionally, plasmashell fails to paint the screen. Remains black with a cursor. Various functions and notifications do occur. Formerly had to restart kdm (and have it still do this) or reboot ... Workaround: Have Yakuake part of active session startup. This will still operate. Hit F12 Kill errant Plasmashell, i.e killall or kill -9 pid of this plasmashell. Now re-run Plasmashell& Home screen will now come up.