Sustainers page highlighting problem

The [Sustainers page](https://adoptium.net/en-GB/sustainers/) scroll highlighting does not work as expected.

The sections for benefit levels (Diamond, Platinum, etc) stay dimmed when fully on the page, they are highlighted when partially off the page, e.g.

Note that the Diamond is dimmed, and Platinum is highlighted. Now scroll further and...

Platinum goes dim, and Gold becomes highlighted.

Open the Sustainers page and reproduce the scroll highlighting behavior across the Diamond, Platinum, and Gold benefit sections. Trace the page's scroll-highlighting entry point; done means the section fully in view is highlighted, while sections only partly off-screen are not.

Written by the indexing model from the issue text.