Unless the current window is in fullscreen mode, the left side of the screen has a 1px dead zone. It spans almost all left side - begins slightly below the top-left corner and ends slightly above the bottom-left corner. It is easy to check as right-clicking in that zone produces nothing instead of a dialog box.

This bug occurs on Manjaro (easy to test as there is a live CD .iso image available) and Arch Linux, it does not occur on Kubuntu 18.04 and 19.04.

I also have found a configuration solution to this bug - setting TouchBorderActivate in section TabBox to nothing in kwinrc file.
E.g. adding below two lines at the end of .config/kwinrc and restarting kwin_x11 solves the problem:
[TabBox]
TouchBorderActivate=

As I don't have a touch screen, it confuses me that the absence of this setting causes the problem.

My current Arch linux software versions:
Operating System: Arch Linux 
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2
Kernel Version: 5.0.7-arch1-1-ARCH