VirtualFlow creates new cells in some instances

Using track of scroll bar new cells are created for the full size of Virtual Flow.

To reproduce issue:
- Run HelloListView / HelloTableView (Any VirtualFlow control)
- Now click the mouse on the track of scroll bar and new cells for the entire size will created.

However if you use scroll arrows or knob to scroll or arrow keys the new cells are created only for 1 cell, vs the full size of VirtualFlow.