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Sometimes in the uPortal mockup file, if a portlet is dragged quickly over another portlet, the drop marker is incorrectly displayed at the end of the column instead of relative to the target portlet.
With the 'greedy' flag on (which it is), the column should get an 'out' event when the dragging cursor moves from the column space into the portlet space, and on this out, we unbind the mousemove handler, trackMouseMovement() (which is what displays the dropMarker) from the column (and bind it to the portlet on the portlet's 'over').
We are getting the out event, and hence we are calling unbind(), but the mousemove handler continues fire, and hence continues to incorrectly display the dropMarker. So it looks like the unbind() is not properly unbinding.