Plugin (or some other way) fixed/sticky side menu for just a section of the page?

Example:

link

If you scroll down past the landing section with the hero picture of an office chair, you will reach the description section. As you scroll down the description, you will see that this section of their page has its own separate left menu that always stays in the top left corner of the screen until you scroll your way out of the section.

This is probably a trick with a fixed-positioned element inside a relative-positioned container, but is there a plugin for this, or a way to achieve this effect from the visual editor (Bakery)?

What would be the easiest, fastest and safest way of achieving this effect?