The code at
https://github.com/wikimedia/mediawiki-core/blob/7313978c1959c716d6a1224b3ec8f4b7ead8134e/includes/Title.php#L3522-L3527
checks if $this->isJsSubpage() or $this->isCssSubpage(), but it doesn't seems to consider JS and CSS in the MediaWiki namespace.
Version: unspecified
Severity: major