I can explain this behaviour in xceed wpf data grid demo.....Steps to reproduce...
1. open this link http://download3.xceedsoft.com/demo/gridwpf/Xceed.Wpf.DataGrid.Samples.LiveExplorer.xbap
2. Click on displaying data.
3. Click on "Ship Via" Column to sort it in Descending Order.
4. Scroll the vertical bar to the top so as to reach the first row.
5. Single Click on the first row i.e. Column "ID" having value 10,249
6. Again click on the "Ship Via" Column to sort it in a Ascending order.
Actual Result: The row is comming at the top and the rest of the grid's space is empty.
Expected Result : The row should come to the last of the grid and and the rows which comes above this last row should be visible.
How can i do this ??