Arrow up from initial infinite scroll paint does not go up a row

When you initially load infinite scroll grid that has more data above, if you are on the first row and arrow up, more data is loaded and you see the scroll thumb move down, but it does not move up a row. you have to subsequently arrow up again to get go up the row.

In this plunker: http://plnkr.co/edit/nhP6CpaL9EJM3orGL41O?p=preview

1. Click on the first row, first column.
2. Hit the arrow up key. Note that the scroll thumb moves down indicating that more data loaded, but it did not go up a row. A subsequent pressing of the up arrow key moves you up the row.