How to reproduce Increase mouse sensitivity or dpi to high (I use 3000dpi on a 12000dpi mouse). Focus desktop. Start a downward-rightward motion with your mouse. As you make this motion, do a single right click without holding down, as you continue the downward-rightward motion. It will activate one of the menu items as though you held down the button. This is not exclusive to the right button. It also happens with the left button on menus instead of context menus. This happens in both Fedora 28 KDE Spin and Kubuntu 18.04 but does not happen in Windows 10 or Gnome 3. It happens with either Oxygen and Breeze widgets and also GTK applications. A left or right un-held click to display a menu should never trigger a menu item. Others in the #kde channel on freenode confirmed this behavior. It is easy to reproduce with my instructions. Both of my mice are new (Logitech g900 and g502). Why has this bug not been apparent up until now? High resolution displays (4K on up) lead users to use higher dpi/sensitivity settings on their mice, especially with premium gaming mice. This is a minority use case that will increasingly become common as 4k takes over.
Further clarification: The mouse release misfire happened independently of menus. Just you only observe the behavior over actionable click targets.
Are you using X11or Wayland?
X11
*** This bug has been marked as a duplicate of bug 393663 ***