I using KDE 5.13 on KDE Neon. I use Left-handed mouse. But When I restart the computer, It goes back to Right-handed even if Settings still shows Left handed.
I have found the solution as follows:
First, create shell script with following line:
xmodmap -e "pointer = 3 2 1"
You can find the documentation on https://help.ubuntu.com/community/MouseCustomizations