On a Kaby Lake i7 (i7-7700) with embedded HD Graphics 630 on Debian Testing with KDE 5.28.0, I had a problem where the window manager (kwin_x11) would fail in the following way: everything would be fine for a few hours or a few days even, but then windows would flicker a bit, and usually much later window contents wouldn’t be updated until I focused a different window and then back, and after a while the whole screen froze and only the mouse pointer kept moving.
To fix a system that is still half responsive, open the run dialog (Alt+F2) and execute:
To fix a system where no windows are updated, switch to the console and do:
DISPLAY=:0 kwin --replace
For a more permanent solution, selecting OpenGL 3.1 in System Settings > Display and Monitor > Compositor worked for me on one system.