From Xojo Documentation
If True, adds a horizontal scrollbar to the ListBox.
The horizontal scrollbar only appears if the sum of the widths of the columns exceeds the width of the ListBox itself. If the widths of the columns is less than (or matches) the width of the ListBox, then no horizontal scrollbar appears.
The position of the thumb is indicated by ScrollPositionX. It is used to scroll the ListBox horizontally without a separate Scrollbar control.