The gadget class inherits from the rect class, and thus all gadgets have a bounding box:
Rectangles

Word for converting from a child gadget's co-ordinate system to a parent's:

relative-loc ( fromgadget togadget -- loc )

screen-loc ( gadget -- loc )

Hit testing:

pick-up ( point gadget -- child/f )

children-on ( rect gadget -- seq )