Requests for comment/Live preview

A proposal to update the code in the UserPreference "Use live preview (experimental)".

Background
We currently have Manual:Live preview as a default part of MediaWiki. It was created in 2007 (?) and hasn't changed much since. There are various newer and better userscripts.


 * Aha! TheDJ is amazingly fantastic as always, and is already working on some parts of this, at https://gerrit.wikimedia.org/r/#/c/131346/ and possibly more in 39272 (Live Preview tracking bug)

Proposal
I suggest merging the best bits of all the alternatives, into a single [gadget/extension/core feature] that can be used at all wikis (WMF and elsewhere).

I also suggest renaming it (because it's not "live" in the sense of concurrent, it's just quicker than default) to "fast preview" or similar.

A table comparing features.