void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
var mousePos = e.GetPosition(scrollViewer);
if (mousePos.X <= scrollViewer.ViewportWidth && mousePos.Y <
scrollViewer.ViewportHeight)
{
scrollViewer.Cursor = Cursors.SizeAll;
lastDragPoint = mousePos;
Mouse.Capture(scrollViewer);
}
}