We already make it impossible to create pages with invisible spaces: http://en.wikipedia.org/w/index.php?title=Horsesho%E2%80%8Be_orbit&action=edit

Is there any reason to do more than this?

(In reply to comment #1)

We already make it impossible to create pages with invisible spaces:
http://en.wikipedia.org/w/index.php?title=Horsesho%E2%80%8Be_orbit&action=edit

Is there any reason to do more than this?

That is some customization on the Wikipedia side (abuse filter presumably). We allow creating such pages in general. For example: http://test.wikipedia.org/wiki/Horsesho%E2%80%8Be_orbit

"Invisible" characters are sometimes needed for typographical reasons. I'm not sure if that is the case for Zero width space, but certainly is for other zero width characters (ZWNJ, etc)- so we should be careful before excluding any such characters.

From the spec - "Zero width space indicates a word break or line break opportunity, except that it has no width. Zero-width space characters are intended to be used in languages that have no visible word spacing to represent word break or line break opportunities, such as Thai, Myanmar, Khmer, and Japanese."

Thus, such characters might potentially be useful in a very very long title, in a language that doesn't use spaces, to indicate where to put a line break in the title. (Of course I don't speak any such languages, so could be wrong on that).

Anyways I don't think we should disallow it without being very sure its un-needed. (OTOH, we do disallow left-to-right mark characters, which are similar, as well as normalizing things like narrow-non-break space to normal spaces, so we do similar things to what is proposed here)

This seems to have been intentionally allowed back in titles in r56918. The commit summary on that revision seems to be a pretty good reason to not strip such characters, so I'm going to go ahead and mark this wontfix.