If the context menu is too big for current screen size, these CSS values are added: overflow-x: hidden and overflow-y: auto, see:
https://github.com/swisnl/jQuery-contextMenu/blob/master/src/jquery.contextMenu.js#L1589

However, this causes that open sub-menus on the left/right of the current main menu aren't visible anymore. I've tried to change it to overflow-x: visible, but it still doesn't work, since it seems that overflow-x: visible is not possible in combination with overflow-y: auto.