There's a bug w/ Q1 2010 (not sure about latest internal build) with SelectionMode = Extended in the GridView. If you try to drag-select with your mouse it doesn't get the selection right (it starts selecting rows far below what you really are trying to select).
You can reproduce this error in your demo app in the "Selection" demo for GridView by changing Selection Mode to Extended. Do that, click a cell and drag your mouse down.
You can reproduce this error in your demo app in the "Selection" demo for GridView by changing Selection Mode to Extended. Do that, click a cell and drag your mouse down.