Droppable box is still droppable even if it's covered by another one

Droppable box is still droppable even if it's covered by another one


As an example is better than any long text, here's my problem :
http://pastie.org/571208
It's also visible at http://lickmychip.com/
As you would see, droppable element is still dropable event if it's
covered by another div.
I tried to fix it with the accept, but it's not the proper way because
sometime the droppable div could be half visible, and because the
':visible' return true while the element is not really hidden
(display:none)
I've looked through documentation about droppable and draggable, but I
didn't found any option that could solve my problem
Is anyone knows how to manage it ?
Thanx
Romain