It isn't possible to access:
https://en.wikipedia.org/w/index.php?title=MediaWiki:Gadget-popups.js
it takes forever.
However:
https://en.wikipedia.org/w/index.php?title=MediaWiki:Gadget-popups.js&action=raw&ctype=text/javascript
(or edit) works properly.
Seems like syntaxhighlight is broken