Steps to replicate the issue (include links if applicable):

• https://commons.wikimedia.beta.wmcloud.org/w/index.php?title=MediaWiki:Common.js&action=edit

What happens?:

You do not have permission to edit this page, for the following reason:
You do not have permission to edit this JavaScript page because it may affect all visitors.

What should have happened instead?:

Interface admins should be able to edit the page?

If I somehow triggered a security feature (no idea how), it should explain what's going on.

Curiously, I can change the visibility of revisions of MediaWiki:Common.js.