Title says it all.
That event is being introduced at #1971108: Convert contextual.js to use Backbone (and support dynamic contextual links).
From #2 of that issue:
Essentially, I want to get rid of this hacky, ad-hoc solution:
// Add "Quick edit" links to all contextual menus where editing the full
// node is possible.
// @todo Generalize this to work for all entities.
- $('ul.contextual-links li.node-edit')
+ $('ul.contextual-links').find('li.node-edit, li.taxonomy-edit, li.custom-block-edit')
.before('<li class="quick-edit"></li>')
This will become more important once #1966704: In-place editing for taxonomy terms and custom blocks lands.