private void ThumbCrop_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
ThumbCrop thumb = sender as ThumbCrop;
thumb?.CaptureMouse();
}
private void ThumbCrop_PreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
ThumbCrop thumb = sender as ThumbCrop;
thumb?.ReleaseMouseCapture();
}