This is very simple to do.
The Navbar links items change their look when they get the Active state. They get the active state when their link target is reached.
To do that, first give an ID to each section.
Then set each link to each section in the link settings panel.
Then give the same class to all links items
Then carefully style the active state for the link: select a link, scroll to the section it links to, notice the active state appearing in green, now you can add your underline.
Once done, scroll to sections to see the effect.
If you work well and set appropriate transitions on the link item, you can achieve some animation on each link.
To have the graceful bar scrolling rom one item to another, it may be a little more complex and require IX2…