Spent a half-day troubleshooting why WikiEditor would not load after upgrading to 1.27. Read through every page of this discussion trying to figure it out.
I finally hit on mod_pagespeed as a culprit. We have mod_pagespeed enabled under apache. We've learned that while a powerful tool, mod_pagespeed causes all kinds of problems. Changes to Mediawiki:Common.css don't take effect, for example. Our solution?
Flush the cache.
We now have a bash script (got this solutions from somewhere else, probably StackOverflow) that we run now whenever we touch something that might be cached (css, js).
1 #!/bin/bash
2 touch `grep "^ *ModPagespeedFileCachePath" /etc/httpd/conf.d/pagespeed.conf | awk ' { print $2; } ' | sed 's/"//g'`/cache.flush
It pulls the location of the mod_pagespeed cache out of the httpd configuration, then touches the cache.flush file, which causes the cache to flush on next page request.
Just flushing the cache caused WikiEditor to magically appear.
Good luck, all!