User story
As a mobile user, I want easy access to the TOC through a floating button
Requirements
Implement a floating button that toggles a TOC overlay on/off.
This ticket is about both the button & TOC container design & placement.
Note that we need to consider interaction with the silly Minerva toast component that appears at the bottom sometimes.
Newcomers might also have the help button in the bottom right corner in that same area on some article namespaces (communities can define which namespaces show the help button in read mode through their local community configuration)
Design
https://phabricator.wikimedia.org/T410325#11413373
Acceptance criteria
- This is no button on pages that have no TOC
- There is a button at the bottom of the page that matches design
- There is a TOC overlay that matches design
- The button toggles the TOC overlay (open/close)
- The button does not interfere with other elements (such as Minerva toast, help button, or other overlays like MMV or ImageBrowsing)
- Nice to have: both button and TOC state accurately reflect url hash (#toc) and update as it changes
