IF elements set to display:none; are dynamically switched to display:block; while within a container set to overflow:auto;...
AND IF the appearing blocks have top and bottom margins...
AND IF the appearing blocks amount to more content than the container can hold without scrolling...
THEN a scrollbar appears, but it is corrupted until moused-over or video-refreshed.
No red should appear in the scrollbar area when the button is pressed.