Compare to the other sortable test. Create the Sortable, then grab a draggable and with the mouse still down, slide your mouse to the left, outside of the window even, and drag up and down. Notice how in this demo the sortable updates irregardless of the fact that your mouse is "outside" of the <ul> tag's bounding area.

Create sortable with ignorePositionX | Destroy sortable | Serialize sortable