Fix dragging helper position when html tag has 'overflow-y:scroll'

http://jsfiddle.net/jPKbL/1/

Scroll page a little and try to move items. 
It happens, when html tag has 'overflow-y:scroll' property and draggable elements have some parents with position: relative.
It doesn't depend on browser.

'overflow-x:scroll' property has same bug.