In development environment, ev.currentTarget is the button.
In Beta Cluster, however, once ext.uls.mediawiki is loaded,
it is now the document (the event has bubbled up). I do not
know what causes the difference, but we can use the target.
Bug: T276255
Change-Id: Ie4c425510293a6f2ce81e4bf4a353f2c8d86d412