private void overviewZoomRectThumb_DragDelta(object sender, DragDeltaEventArgs e)
{
//
// Update the position of the overview rect as the user drags it around.
//
double newContentOffsetX = Math.Min(Math.Max(0.0, Canvas.GetLeft(overviewZoomRectThumb) + e.HorizontalChange), DataModel.Instance.ContentWidth - DataModel.Instance.ContentViewportWidth);
Canvas.SetLeft(overviewZoomRectThumb, newContentOffsetX);
double newContentOffsetY = Math.Min(Math.Max(0.0, Canvas.GetTop(overviewZoomRectThumb) + e.VerticalChange), DataModel.Instance.ContentHeight - DataModel.Instance.ContentViewportHeight);
Canvas.SetTop(overviewZoomRectThumb, newContentOffsetY);
}