What makes you think that the scrollbars shouldn't flicker ? I see the flickering with Firefox 17, Seamonkey trunk, Opera12.11 and Safari6.0.2(OSX)
The scrollbars appear when the circles go beyond the window. If the circles did not move after moving outside of the currently viewable area, the scrollbars wouldn't flicker -- they'd remain in place for you to scroll up and see those hidden circles. Consequently, this appears to be expected behaviour.