For my Firefox desktop, even the "beautiful solution" at the end highlights "Middle Section" even if the Conclusion is fully visible, but I'm just not quite at the bottom of the page in terms of scroll.

Surely the answer is to highlight all onscreen anchors. You don't know where my eyes are looking on a page with two headings on it.

Highlighting the heading to scroll to is a must.

It’s what headings maps extension does when you click on one [0].

[0]: https://addons.mozilla.org/en-US/firefox/addon/headingsmap/