after many weeks searching for a solution (in the available spare time :-P), a colleague in the Manjaro forum gave me a big hand in understanding how to disable the shadows around menus and windows in Manjaro JWM. :-)
here's the full solution.
and bellow the short answer:
remove the following line in the startup script (key 8 in JWMConf)
<StartupCommand>xcompmgr -c &</StartupCommand>