The signature of the function is func(self, arg1, arg2, checked). Arg1 and arg2 are user defined variables and can be literally anything but unlike most OnClick functions, the button state is not passed!
In fact, right button clicks are not recognized at all, meaning, when I right-click a button nothing happens. It only recognizes left-clicks and I'm trying to change that.
Each menuList table has a specific signature more on which can be found here:
UI Object UIDropDownMenu and
EasyMenu