When you have a relatively large number of records in a XamDatagrid and use the vertical scrollbar to scroll to the end of the list, the scrollbar's thumb becomes longer.

If you scroll back up to the beginning of the list, the thumb shrinks again.

Can be reproduced in the xamFeatureBrowser.

1) Open the example (under xamDataGrid > Layout and Behavior) called "ExpandableFieldRecord Display Modes"
2) Expand the first item in the list, "Thriller"
3) Resize the xamFeatureBrowser window so that it is small enough that a vertical scrollbar appears in the datagrid of the example
4) Scroll to the bottom of the datagrid and look at the length of the thumb
5) Scroll to the top of the datagrid and look at the length of the thumb

(This is not a major problem, but some of our users have become distraught after scrolling to the bottom of long lists and seeing the thumb grow, thinking that some of their data had been lost.)