Sortable list connected to a list in a "position:fixed" does not drag properly if the two lists overlap

Same issue as reported at http://stackoverflow.com/questions/4092817/dealing-with-overlapping-jquery-sortable-lists, which provides a minimal repro.

I've duplicated the repro from Stack Overflow into http://jsfiddle.net/GPZVZ/. To demonstrate the error, scroll down so that the two lists overlap. Now drag an item from the "position:static" list over the connected list. The item still interacts with its source list.