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?
I guess I figured it out, but it is strange.
A couple of weeks ago, I noticed that one could use OpenGL3.1 as a compositing type, so I switched to it. I ran my computer like this for a couple of weeks and suddenly, last night, there was a strange flash and suddenly the screen edges no longer worked, windows made an odd flash as they appeared and other stuff. I noticed that the edge effects were greyed out.
I investigated further a moment ago and noticed that I had missed, last night, that desktop effects had turned themselves off. I tried to reenable them, but they wouldn't turn on. Then, I switched to OpenGL2.0 and tried to reenable desktop effects, and now it all seems to work like it should.