replace the event creation and the dispatch line by :
```javascript
var event = new MouseEvent('click', { bubbles: true, cancelable: true }); // closer to the real stuff
this.sinon.spy(event, 'stopPropagation'); // does it work ?
ThreadUI.contactPickButton.dispatchEvent(event); // use the event you instantiated
sinon.assert.called(event.stopPropagation);
```

Another idea is to add a click event listener on a parent, and see whether it's called. But spying on stopPropagation works for me.