@mmcky I reckon it's pretty useful functionality. Almost like the minimap in code editors. I'd be happy to implement.
This would include:

• Make the whole in-page-toc sticky (not just 'back to top')
• Add active highlight to toc items depending on page scroll

Yeah it is pretty good. Now that we are inheriting from sphinx-book-theme are we just overriding the styles? Or do we need to implement this in our context?

I'll take a closer look. There might be a tiny little work to refactor the markup so that it can use the JS/CSS from SBT.
Hopefully as you have suggested all we'll need then is some QBT styling.
@mmcky Shall we go ahead and implement this across the quantecon-book-theme sites?

Yeah let's see what it looks like. I think the animation and navigation is nice but John might like the simplicity of what we have. So let's make a PR and get comments :-)

We could always have a simple-contents-toc=True/False flag