There was a small bug in the code where the draggable unit would be displayed behind other draggable units or droppable units. The draggable unit should always be on top of other elements on the page.

The following code raises the z-index of the draggable at the start of the drag and returns the z-index to normal once the drag stops.

$(div_drag).draggable(
{
	start: function( event, ui ) {

		// Make sure the draggable unit is on top
		$(this).css('z-index', '100');

	},

	stop: function( event, ui ) {

		// Return the z-index back to normal
		$(this).css('z-index', '1');

	}
});