Vertical scroll moves when scrolling horizontally

I have a grid dealing with a large dataset. It renders as expected. Grid contains 25 columns. I have row-selection set to true and showing 25 rows for page-size. Vertical and Horizontal scrollbars are enabled. Grid shows first 10 rows and vertical scrollbar to get to remainder. When I scroll down and select the 2nd or 3rd row from the bottom and scroll to the right to see other columns - when I get to about the 18th column the vertical scrollbar moves up.

If I set page-size to 10 the shifting doesnt happen.