I'm not sure we should implement collapsible sections for desktop.
If built and instrumented, one may find that users use it a fair bit and it may be better-than-nothing as solution for certain use cases. But I don't think collapsible sections would be an adequate feature for those use cases.
Our table of contents is in desperate need of improvement. Having that be more accessible throughout the reading experience would be a big step forward[1] (much like the Wikipedia iOS app). Having a proper TOC means users don't have to collapse/expand anything.
This would allow users to have a birds eye view of the document at all times, jump to any section at any time, whilst still being able to scroll through the document top to bottom as one would expect.
>From a performance viewpoint (as opposed to usability) we can still do optimisations such as not loading images until a section is accessed (lazy-load).
-- Krinkle
[1] Different ideas around an "aside"-accessible table of contents:
* http://underscorejs.org/
* Wikipedia iOS App: http://i.imgur.com/Sg0pqsg.jpg
* User manual: http://asciidoctor.org/docs/user-manual/