We are currently unable to update OOjs UI library in MediaWiki core without forcing V+2, because updating PHP and JS versions requires two commits to two repositories, and tests immediately fail upon version mismatch.
- https://gerrit.wikimedia.org/r/#/c/187854/ in mediawiki/core to update the JS distribution used by ResourceLoader (and composer.json)
- https://gerrit.wikimedia.org/r/#/c/187855/ in mediawiki/vendor to update the PHP distribution
When one of the changes was force-merged, tests for the other one passed.
I resolved the immediate issue by force-merging, but this will be a problem again in a week or two when there's another OOUI update.