Grouping in DataGrid

Hi,

I am using grouping in datagrid. Everything works as expected, but when user selects UP/DOWN in scroll viewer, it does not move one row at a time. Half row is shown for one down and then user has to press down again to show one full row. 

 I used  <Setter Property="ScrollViewer.CanContentScroll" Value="True"/> but it has not affected. When there is no grouping it works as expected, which means it scroll one row at time.

Thank you.