When a scrollbar is on the screen for a web page, clicking on the bar and
dragging it can be used to scroll, but it will remain active if the button
is released over a toolbar, the menubar, the tab bar, or the security icon
in the lower-right corner.  Basically, the mouse-button-up event never
reaches the scrollbar, so it continues to follow the mouse (and scroll) as
if the button was never released...