With Plasma 5.10.0 from ArchLinux repositories, I noticed some unresponsive clicks on fullscreen video games, such as Xonotic 0.8.2 (SDL executable). By default, one touchscreen left edge action was configured. After disabling this action and restarting kwin, the issue is gone, no more mouse issues with fullscreen games.
Sorry, I forgot to mention that I use the X11 Plasma session, with xorg-server 1.19.3 and nvidia 381.22 drivers.
*** This bug has been marked as a duplicate of bug 380476 ***