The new input event type "auxclick" introduced here provides web developers a way to listen to the click behavior of non-primary buttons, and potentially cancel their effect (eg. opening a new tab when middle clicking on a link). This new input type is needed because the "click" event is restricted to primary button only according to the spec https://w3c.github.io/uievents/#event-type-click. So after this change "click" event will no longer be dispatched for non-primary buttons.
Docs: https://developers.google.com/web/updates/2016/10/auxclick