Thank you for contacting Telerik support.
I would like to clarify that this example does not support scrolling. You can modify the problematic line of code to search only in visible items:
int UpperBound = 0;
UpperBound = visualReplacedItem.Bounds.Height * targetListBox.ListElement.ViewElement.Children.IndexOf(replacedItem.VisualItem);
I hope this helps. Do not hesitate to contact us if you have other questions.
the Telerik team