In a middle of KDE desktop session (as run by `/bin/sh /usr/bin/startkde') the
task bar at the bottom of the screen disappeared without any message at all,
let alone any advice as to how to get it back.  Along with main menu button
({Application Menu}, {A launcher based on cascading popup menus}).  Moving
pointer around in left upper corner of the screen sometimes would yield a
choice of images of top- level windows, including ones previously iconified by
user, scattered on the screen.  But that is all.  Until re- boot the task bar
never re- appeared.

If it is still a feature triggered by some specific user gesture (key
combination, pointer movement), how do we figure such a gesture, and how do we
get the task bar back?