I would imagine that in "closed position", the mechanism actually slips "down" a bit (the clcik you would feel when closing it) and then when opening it, the slider mechanism actually jumps "up" a bit (half of the radius value of the fillet on the top pieces underside edge) and then tracks along the slide.

Maybe I dont actually follow what your describing? If you post 2 sample objects configured like this I would love to look with you.