There are many upstream bug reports describing the same (yes, I have been struck as well).
People in https://bugs.kde.org/show_bug.cgi?id=355410 suggest to remove the x11 libinput driver and instead install the x11 evdev driver. Here (Debian Sid), both packages were installed (xserver-xorg-input-libinput and xserver-xorg-input-evdev). Removing the first and restarting KDE solved the problem for me. $ sudo apt-get remove xserver-xorg-input-libinput That also removes two or three other packages, so be careful. The KDE people say, it's likely a QT5 bug... just a hint for the maintainer.
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