This is a great snippet by Hakim El Hattab where you can see a scroll progress indicator that highlights sections of a page that are currently in view. You have a navigation menu on the left sidebar and as you scroll there is a progress indicator that moves along the menu to indication which section of the page is active. This will work great when you have a long form article.
If you are looking for a scroll indicator in the top nav bar check out this snippet.
If you are having trouble with the pen, try the archived copy on GitHub