Using the source inside GitHub, when the number of rows displayed doesn't enable the scroll bar, it also fails to enable the CSS formatting. The reason for this (as pointed out by Larry Woods' comment on February 4, 2011 at 7:49 pm) is that the code has this entry which doesn't seem to be working correctly:
// Bail out if there's no vertical overflow
if ($(this).height() <= settings.height)
{
return this;
}
If these rows are commented out it appears to work well.