As reported here with screenshots, the behavior on live does not match the "current behavior" screenshot at the top right of this ticket. The text there reads "Currently a section is considered active (and therefore bolded in the table of contents) once the top of it's heading begins to intersect with the top of the viewport (or the bottom of the sticky header).", but in fact even this is not working — the bolded section is often not visible on the screen at all. I'm not sure if I should be starting a separate bug for this, since it seems to be a separate issue that won't be fully resolved by implementing the primary feature described here, but it's also similar enough that it could probably be addressed at the same time.