This feature will not work with the load on demand feature. Virtual paging loads entire rows and the load on demand loads partial rows. Virtual paging requires that EnableClientScript is true. For the best performance, the EnableAjaxCall property should be true.
The height of the thumb bar is related to the total row count.
FpSpread1.Sheets[0].AllowVirtualScrollPaging = true;
FpSpread1.Sheets(0).AllowVirtualScrollPaging = True