Gentoo x86_64; Kernel 4.9.6; 2 screens; multiple active desktops; nvidia TwinView
ClusterSSH (cssh from commandline) is a utility to multiplex the keyboard to multiple xterm windows.
Upon first keypress into the cssh input window, all windows can no longer be resized or moved. Normal operation otherwise.
Moving and resizing can be reinstated with a 'kwin_x11 --replace', until next keypress into the cssh input window.
Happens in all versions over plasma 5 I've encountered -- it was worse (crash crash crash) in earlier version.
*** This bug has been marked as a duplicate of bug 348569 ***