I have noticed this several times when accessing parallel resources. It appears on the first entry of the list that the edge of a book is encroaching from the left. See screenshot below.
That's an indicator meant to show you which book is currently selected. It's ugly and orange because we haven't yet changed the color from what it was in the Windows product.
Oh! <blush> I erroneously called a feature a bug! My apologies! [:$]
