Have used Xmodmap successfully with 14.04 starting up with startup app.
Not so with 16.04. It does work when I activate it directly. Does anyone know of any change from 14.04 to 16.04 that causes this problem. Tried different approaches.
open gnome-session-properties. add command and name. desktop file will be created in ~/.config/autostart.
remenber: use absolute path in the command line, eg /home/xxx/.Xmodmap. never use '~' as your home dir.