Now that I finally have JS previewing for wikicode working a bit better...
I present a new user script:
https://en.wikipedia.org/wiki/User:TheDJ/ActualLivePreview.js
If your window is wider then 1200px, it will split it in half and show you
your edit surface and your content preview side by side, automatically
updating the preview !
How cool is that :)
Please test it, and let me know, if you think we should add something like
this to core !
DJ