MobileFrontend provides the MobileUI class (https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/includes/MobileUI.php) which provides helpers for rendering mediawiki ui components.
It uses it, but Minerva also needs access to it to support a desktop skin.
OOjs UI is not an option in Minerva at the moment. This is too much work and would have negative consequences on first paint - the Minerva skin only needs to render buttons and icons... no other widgets.
- MobileUI should live in core and be renamed MediaWikiUI
- We copy and paste MobileUI into Minerva and rename to MinervaUI