Would a machine-generated summary of a section plus the section title be a sufficient navigational affordance for readers without expanding all sections by default?

Reminds me of old-timey chapter titles like this one from Mr. Midshipman Easy, by Frederick Marryat (1836), via Project Gutenberg and Metafilter:

CHAPTER I Which the reader will find very easy to read.
CHAPTER III In which our hero has to wait the issue of an argument.
CHAPTER XII In which our hero prefers going down to going up; a choice, it is to be hoped, he will reverse upon a more important occasion.

Quick sketch

ToC would work better as a modal because we already have a floating sticky "bar" in the design, and we don't want two floating elements on top of eachother. You also don't need to scroll or read main article content while you're looking at the ToC.

Related, possible dupe with T365631

Web team reviewed this today in quarterly grooming and this task is still valid and should be retained in backlog.
This is going to get folded into KR work at some point.

Assuming this lingering task with no active project tags is about the MinervaNeue codebase. Please add codebase project tags to tasks.

Downloaded the Android app for the first time and thought of this! I love having the floating actions bar at the bottom of the screen.

I struggle to use the Android app, though, because it doesn't handle tabs from external apps (e.g. Google search) in the background the way, say, a mobile web bubble browser can.

In a perfect world, logged in users on mobile web could customize a similar actions bar, but I think a default that includes things that are otherwise hidden on a small screen (like TOC) would be great and a lot simpler to start.