Thanks for writing.
You can use the ScrollStateChanged event of RadDataBoundListBox. In this event, you can listen for the scrolling state. While the list box in a scrolling state you have to start a timer that ticks at some interval (500 ms for example). In the timer tick handler you can check the ViewportItems property to see which items are in the list box viewport and mark them as read.
I hope this helps.
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