OSX doesn’t show scrollbars for a while. Although this saves another 16px of space it could be hard to determine how long you need to scroll for the end of the page. Zachary Olson came up with a clear idea: use small dots to indicate the scroll position on a page.
The dot stick to a header in the text. When the header is out of the screen it will stick at the top of the container. The same goes for the headers which are still to view, these stick at the bottom of the container.