One more remark: I've only observed the effect when the OpenGL 3.1 compositing backend in kwin is active. I tested with OpenGL 2 backend over the last week and have not seen this happening since. I should also mention that I've had the egl interface activated, which is not recommended for kwin. I've not had issues with it before, but it could be related, so the next thing I'm testing is glx/OpenGL 3.1 and hope I can narrow this down this way.