private void Window_MouseDown(object sender, MouseButtonEventArgs e)
{
//
// Update the position of the overview rect to the point that was clicked.
//
Point clickedPoint = e.GetPosition(content);
double newX = clickedPoint.X - (overviewZoomRectThumb.Width / 2);
double newY = clickedPoint.Y - (overviewZoomRectThumb.Height / 2);
Canvas.SetLeft(overviewZoomRectThumb, newX);
Canvas.SetTop(overviewZoomRectThumb, newY);
}