I’m running KDE on two different systems, and one of them exhibits the following problem very often, and the other just did for the first time:
Windows stop updating their content, and perhaps flicker a bit. Switching to a different window and back causes the window contents to be updated, but still frozen. Which means that the application itself is not crashed.
The following command fixes this:
You can run this from the run command prompt (Alt+F2) (also called Plasma search or krunner), or you could run it in a terminal. (You’d have to make sure the process doesn’t exit when you close the terminal though.)
If everything appears to be frozen and you can’t get to the run command prompt, you could still switch to a console, log in, and try running the following:
DISPLAY=:0 kwin --replace
Both systems have internal Intel graphics (quite different chipsets though) and KDE5.
The above commands will fix the problem for that time. Your open applications should not be affected by the change. I haven’t looked much into permanent fixes, but changing the rendering backend (System Settings → Display and Monitor → Compositor) may change the frequency the problem is triggered or maybe even get rid of it altogether. (I felt that OpenGL 2.0 probably triggered the problem fewer times than OpenGL 3.1.)
I’ve noticed a fair amount of traffic to my KDE-related posts. If you run into any weird KDE problems that you don’t know how to fix, feel free to leave a comment and ask.