With feeds that have very short items (i.e. excerpts - bbc.co.uk for example) the first item is often not highlighted as I begin to scroll so remains unread. Sometimes the last item or two don't get highlighted as they obviously don't reach a high enough point in the preview pane.
Related: if there are only two or three short items and therefore no scrollbar there's no way to use this feature to mark the items read.
Just a suggestion: the feature doesn't seem to take any account of where the mouse cursor is. I find that I have the cursor somewhere around where my eye is focused. Sometimes the yellow highlight jumps to the next post, but I'm still looking at the one before. When I do reach the end in a long list of small feeds I instinctively move the cursor down over the last items (because the sroll has stopped). You could use the cursor position as a guide rather than simply the vertical position of the post. Love the feature though!