Not sure I follow. The typical way is to select from listA and then via button or double click, move selected item(s) to listB, thus listB would not need to be scrolled. Nor would you need to scroll listA to deselect; you'd do the opposite - select from listB & button click or double click items to move them back to listA. The advantage is that you can also use buttons to Move All:
ListA >> ListB
ListA << ListB
or move selected from/to either list:
ListA > ListB
ListA < ListB
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.