A strange thing happened.
I opened libreoffice and plasma crashed. This is a regular occurrence
since about 10 days that I had hoped would disappear with the arrival
of kde-4.11.1, but it didn't. I closed libreoffice and tried to
switch to another virtual desktop, when I noticed that screen edges
no longer work.
I just updated the system and a new kde-workspaces came, so I thought
this just might be the fix, so I didn't worry about it. I restarted
(not off and boot) the computer, expecting everything to be fine. It
isn't.
I checked systemsettings and the edges are set to active windows and
desktop grid, but they are shown greyed out, so I thought that
somehow these effects must have become disabled. They didn't. They
are shown as enabled.
I logged out and back in. Still the same.
Ideas?