The user selector used in the groups and capability pages that is located in user/selector/module.js is in desperate need of migration from Yui to a JS module.
It is currently blocking any improvements to these areas / pages, since (rightfully) any changes to the YUI code just adds more tech debt to the migration.
For e.g. it has blocked MDL-51202
I have mapped out the YUI dependencies from this file (see attached diagram) and can see it is not a trivial task.