Elements are still droppable while they're in the hidden part of a scrollable container

Elements are still droppable while they're in the hidden part of a scrollable container


As an example is better than any long text, here's my problem :
http://pastie.org/571208
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