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I'm trying to understand this interaction

I’m trying to understand how this is done where as the user scrolls through the form, items in the left bar are highlighted.

https://preview.webflow.com/preview/jobstemplate?utm_medium=preview_link&utm_source=dashboard&utm_content=jobstemplate&preview=78af3251fffd308778711c64ae18da00&mode=preview

If you look at the preview you in the designer you will see those ‘items in the left bar’ are links. Look at the style for the ‘Current’ state of the link and you will see the transistion effects and colour added to create the highlight.