Thi is in the (most likely very rare) case that someone has just the right amount of items in the treebar so that the last item will touch the very bottom of the window without any scrollbars showing.
Another requirement is, that at least one branch is collapsed. If that branch is then expanded, scrollbars will be shown (naturally).
Now if you collapse that very same branch again, the scrollbars will still be visible.
After collapsing and expanding another branch, the scrollbars will be gone and the list will again be completely filled.
...as i said, it's really minor