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
Scroll Progress Indicator Snippet
See the Pen Progress Nav by Hakim El Hattab (@hakimel) on CodePen.0