private bool IsUserVisible(FrameworkElement element, FrameworkElement container)
{
  if (!element.IsVisible)
    return false;
  var bounds = element.TransformToAncestor(container)
    .TransformBounds(new Rect(0.0, 0.0, element.ActualWidth, element.ActualHeight));
  var rect = new Rect(0.0, 0.0, container.ActualWidth, container.ActualHeight);
  return rect.Contains(bounds.TopLeft) || rect.Contains(bounds.BottomRight);
}