Page TOC sidebar highlight doesn't work when the section content is very long

If a section is very long and no headings are present then the highlight doesn't work. Maybe, the highlight should update only if there is a new section to highlight (keep memory, or update the classes only if there is a change) and keep the old/current one otherwise.

Relevant file: `on-this-page.js`.

https://github.com/user-attachments/assets/b3ddbc66-9a11-4121-a33c-1b776f1df24e

Start by reading on-this-page.js and reproduce the TOC behavior with a very long section that has no headings. The fix is complete when the current sidebar highlight remains active until a new section is available to highlight, rather than disappearing or changing incorrectly.

Written by the indexing model from the issue text.