Same here with OS10.10, DtPO 2.8.1 in 3-pane view, sorted for “modified” with the latest items on top of the list.
It’s only the last most recent single item that I have to scroll for to see it. It is constant no matter if the item is read or unread. You can also click it, switch to another feed and back again and it will not be shown.
And yes, good point, see above: it is only when the preview window lists so many items, that the scroll bar appears.