I think that idea is related to Discovery-ARCHIVED: it's a way to have an easier access to contents.

So far, Discovery-ARCHIVED has focused on finding and navigating pages, rather than finding content within a page you're already on. I am not in a position to say whether this should be in Discovery's scope or not, but it hasn't been.

It seems relevant to Reading (rather than Discovery), but I think design is a good starting point.

Pasted from T173812:

I think every the version of Wikipedia apps (at least on Android and iOS) got some sort of floating TOC column, so it's probably a much needed feature and standard.

Since it may be usefull to see if it should be a UI-standard, I'm adding the releated project so they can keep tabs on this task.

In MediaWiki, aspects such as the table of contents are considered the skin's responsibility. I understand that those who like this feature to be in the sidebar want it to be there as option in all possible skins, but that is not realistic and is contrary to this separation of concerns. And while it might smooth innovation in the short-term (if we consider this an improvement) it also impedes innovation in other directions.

This should be reframed as a feature request for 1 specific skin, e.g. Minerva or Vector. Or, if interested more in the general availability for Wikipedia, I'd recommend untagging the skins and letting Reading-Web decide if and how they might bring this experience to life.