This works more or less the same way the current beta on Android does. Once the user starts scrolling up, the floating button appears, and they can tap it to jump back to the top of the screen
Open prototypeIf any sections are open the floating button is present. Tapping the button closes all sections and scrolls to the top of the sections list.
Open prototypeSection headers are sticky when sections are open, making it easy to collapse the current section and then jump into a differenent section. One argument for this is that it doesn't introduce any additional elements on the page.
Open prototypeSimilar to what we do on iOS and Android, as well as a feature available on MediaWiki. Users can easily access a table of contents for the article and jump around. I don't love how this is duplicative of the section collapsed headers, which are effectively already a table of contents.
Open prototypeCombination of the two above concepts, wherein section headers are sticky and when stuck turn into triggers for the toc menu.
Open prototype