Show the current anchor link in the ToC sidebar (scrollspy)

It's true that in pages where there are a lot of items, having two scrollbars in the Table of Contents sidebar (on the right) can be confusing.

Docker docs have this nice functionality where the heading in the sidebar is highlighted as you move, for example https://docs.docker.com/get-started/#containers-vs-virtual-machines.

Edited Jan 30, 2019 by Marcia Ramos