Online documentation (user guide) sidebar broken

Summary

The sidebar in the user guide is no longer functional. It doesn't show the headings or links to pages making it very difficult to find what you need, as you have to guess search terms. It stays collapsed listing only the main page no matter where you navigate or what you search for.

Steps to reproduce

Just go to the online user guide and try to navigate through.

Have reproduced on Edge, Firefox, Chrome.

What is the expected correct behavior?

The sidebar should populate with the document structure as it does for the API guide. It did so up until about 2 weeks ago. Like this: