you could expand the interface area to cover entire scene width/height...
also you could add a query to the left mouse button actions inside of the mouse properties section of the game tab.
for example: inside of "start following action" for left mouse button....
if condition "inventory_open?" is true
if current object does not exist
character > stop character
on left click: if no object (item, character, object) exists under mouse cursor then it must obviously be a new character destination. & so we cancel it out with the stop character action part.