I set up a mediawiki.ui test instance on Labs. For now, it is dedicated
to showing the in-progress patch which is rolling out mw-ui to all
buttons (
https://gerrit.wikimedia.org/r/#/c/52169/). You can see what's
implemented so far in the commit message in the top right.
The URL is
http://mwui.wmflabs.org/wiki/Main_Page . Feel free to check
out those pages, create accounts, make test edits, etc. However, it's
not a wiki for actual documentation or design mockups, etc.
After that patch is done, the instance can be used for testing other
mw.ui work.
Matt Flaschen