Sidebar TOC: bold/highlight the current section while scrolling

What

On pages that include a sidebar table-of-contents / in-page navigation (list of headings), it would be helpful to visually highlight the currently active section as the reader scrolls.

Why

On long pages it’s difficult to tell where you are in the document. A clear active state improves navigation and readability.

Expected behavior

• As the user scrolls, the sidebar item corresponding to the heading currently in view becomes bold (or otherwise clearly “active”).
• The active state updates as the user enters the next/previous section.
• Clicking a TOC item should also set the correct active state.

Actual behavior

• Sidebar items don’t clearly indicate the current section while scrolling in the Table of Contents located in the sidebar.

Suggested implementation

Add scrollspy behavior (IntersectionObserver) to toggle an active class based on headings in view.

No file or test is named. Start by locating the sidebar Table of Contents implementation and the code that renders or links its headings; then trace how scrolling and TOC clicks are handled. Done means the heading in view has a clear active or bold state, the state changes between sections, and clicking a TOC item selects the corresponding section.