Update TOC highlighting code to consider the user 'on' a page section before it reaches the top of the viewport

Following on https://gitlab.com/gitlab-com/gitlab-docs/issues/141, I find that as I'm scrolling through a page and reading naturally, I start to read a new section when its header is around 1/3 of the way from the top of the viewport. Once I start reading, if I glance at the TOC, I notice that the header highlighted there is still from the "previous" section, even though only a small bit of that section is still displayed in the viewport.

Is there agreement with the idea of updating the TOC highlighting not when a header reaches the top of the viewport, but before that?

Is this possible to implement? How much effort is required?