Does not work with an indexed scrollwheel -- one click of the scroll wheel moves like 4 lines in your list, which seems to break the assumptions of your code. I get very strange behaviors, e.g. "pick" Five, scroll up and down, see swapping of Ten and Six during scroll. Then "place" results in Five being in the same place as before but Nine and Ten are swapped. Similar when using arrow keys.
The ones that are moving the wrong item to the wrong place are fixable, but there will always be a problem when scrolling is quantized if there is a possibility that any item in the list is shorter than the scroll quantum.
