On Windows, if you use the up/down arrows that appear in a drop down menu, when reaching the first or last item, the arrow no longer turns blue, providing no visual indication of a click. (Use the "Component" menu of this bug report to see the issue). This is potentially a platform integration issue as there are some platforms that disable the up/down arrows when at the edge, but in the case of SafariWin, it should be done in order to remain consistent with all the other scroll bars it draws. This could also be a SafariTheme issue and/or a general issue relating to platforms that use pop down menus.
I should clarify that this happens not specifically when at the first/last item, but when no scrolling is necessary (scroll section is at the very top/bottom).