Jon and I have been discussing how such an extension would look like in the mentioned
task, as there is a requirement to change MediaWiki to make the category browser possible
as an extension. However, as I mentioned in that task, if no one is actually interested in
such a feature, I'm not going to bother spending time working on it.
So, using that opportunity; if you (or anyone else) can't live without this feature
and would like to see it return in an extension, please let us known by commenting on the
task.
Mainframe98