Sortable: Wrong placeholder placement with float:left

I have a ul with sortable enabled. The li elements are styled with float:left. However, when dragging an item from the second row to the one above (in case a li wraps to the next line), the placeholder is occasionally placed wrongly. Other times is will be placed faster, resulting in a flicker.

This is a jsFiddle I set up: http://jsfiddle.net/3vzCn/3/

1) Resize your window so that Try to drag the last li (foo4) is placed on a new line.

2) Drag foo4 to foo2.

3) Sometimes at this point the placeholder is not at foo2 but at foo3. When one lets the mouse stay there, the placeholder will automatically be placed at foo2 after a second or so.