This property, when set, causes the list box to display the line number of the item on top of the list box, scrolling the latter if necessary. Items are numbered sequentially from the top of the list, starting at 1.
Setting THUMB-POSITION to a value greater than the actual number of lines in the list box or to a value less than 1 has no effect. When queried, this property returns the line number of the item currently displayed on top of the list box.