vis.js Timeline - mouse events issue

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/vis/4.17.0/vis.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/vis/4.17.0/vis.min.css">
<div id="visualization"></div>

CSS

/*
  Steps:
  1. open the console
  2. click on the groups labels (First, Second, Third)
  3. was the 'mousedown' event logged in the console?
*/

JavaScript

var groups = new vis.DataSet([
  {id: 0, content: 'First', value: 1},
  {id: 1, content: 'Third', value: 3},
  {id: 2, content: 'Second', value: 2}
]);

// create a dataset with items
// note that months are zero-based in the JavaScript Date object, so month 3 is April
var items = new vis.DataSet([
  {id: 0, group: 0, content: 'item 0', start: new Date(2014, 3, 17), end: new Date(2014, 3, 21)},
  {id: 1, group: 0, content: 'item 1', start: new Date(2014, 3, 19), end: new Date(2014, 3, 20)},
  {id: 2, group: 1, content: 'item 2', start: new Date(2014, 3, 16), end: new Date(2014, 3, 24)},
  {id: 3, group: 1, content: 'item 3', start: new Date(2014, 3, 23), end: new Date(2014, 3, 24)},
  {id: 4, group: 1, content: 'item 4', start: new Date(2014, 3, 22), end: new Date(2014, 3, 26)},
  {id: 5, group: 2, content: 'item 5', start: new Date(2014, 3, 24), end: new Date(2014, 3, 27)}
]);

// create visualization
var container = document.getElementById('visualization');
var options = {
  //groupOrder: 'value',
	groupOrder: function (a, b) {
    return a.value - b.value;
  },
  groupOrderSwap: function (a, b, groups) {
    var v = a.value;
    a.value = b.value;
    b.value = v;
  },
  // editable: true,
  groupEditable: {
  	order: true
  },
  orientation: 'top'
};

var timeline = new vis.Timeline(container);
timeline.setOptions(options);
timeline.setGroups(groups);
timeline.setItems(items);

var onMousedown = function(e) {
	console.log('mousedown event:', e);
};

var labels = document.getElementsByClassName('vis-label');

for (var i = 0; i < labels.length; i++) {
    labels[i].addEventListener('mousedown', onMousedown, false);
}