Hmph, somebody used that? It was undocumented and not intended for public consumption, Jon and I removed it a few weeks ago.

Yeh. it's also styling this on 2 ids :-S

This is a good example of why mixins without parameter names are bad - this causes 3 sets of duplicate css.

The url shortner should be using mw-ui-button and mw-ui-input [1] when available.

[1] https://gerrit.wikimedia.org/r/149173

So, what's the migration plan? Undocumented or not, it ended up in the 1.23 release, so it got used (the extension is live on 2 wikifarms at least).

(In reply to Jon from comment #2)

The url shortner should be using mw-ui-button and mw-ui-input [1] when
available.

[1] https://gerrit.wikimedia.org/r/149173

Linked change hasn't been merged yet, so not really useful...

This line of css should be removed. I'm not sure why it was using that mixin in the first place.

I suspect it was trying to style an input in the mediawiki ui style.
If it wants to use that style it will need to wait for that patch to be merged so help getting it merged appreciated.

Is that UrlShortener using a CSS definition that get removed from core?

Change 149902 had a related patch set uploaded by Jdlrobson:
Fix LESS exception

https://gerrit.wikimedia.org/r/149902

Too much talking :P
Above fixes it.
There are follow ups which can be merged without dependencies that fix the styling.

Change 149902 merged by Bartosz Dziewoński:
Fix LESS exception

https://gerrit.wikimedia.org/r/149902

Thanks :)