Interface: Sortable lists get a blank-looking DIV "sortHelper" as a visible row

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In the demo at ​http://interface.eyecon.ro/demos/sort_lists.html and in my own use, I've found that dragging a LI up above the first LI, then dropping, causes a blank line to appear. Sometimes further re-ordering causes this blank line to move around within the list. The Firefox DOM Inspector shows this to be a DIV called "sortHelper" inside the UL. It is styled as "margin: 0px; display: block; width: 300px; height: 16px;". Not sure why this style is needed on this DIV. Using jQuery to hide it works after the fact, but it reappears visibly later as more user re-ordering is done. Using jQuery 1.1 and Interface 1.1.