User story
As a mobile user, I want to keep oversight of where I am at while I am scrolling throughout an article
Requirements
The sticky header always reflects the name (and edit link) of the (sub)section (or the main page title) whose content is currently at the top of the mobile screen.
"section" here means more than only the h2's (as was the case for the earlier sticky headers prototype), and refers to all levels of depth, i.e. everything shown in TOC.
Design
https://phabricator.wikimedia.org/T410325#11413314
Acceptance criteria
- Title & edit link in the sticky header are accurately updated once the relevant section title crosses the top of the viewport
- For both upward and downward scroll
- For all depths of titles, from main page title down to small subsections
- The "subtitle" (short description) is only used for the main page (page title) section
- In TOC, the section currently at top of the page shows as bold
