title: Mapbox GL JS Listen for popup open event?
body: <p>Is there any way to detect when a Mapbox popup gets opened? <a href="https://github.com/mapbox/mapbox-gl-js/issues/2584" rel="noreferrer">This issue</a> in the mapbox repo describes a <code>close</code> event that works like this:</p>
  
  <pre><code>let popup = new mapboxgl.Popup()
  popup.on('close', function(e) {
      alert()
  })
  </code></pre>
  
  <p>However, I can't find any way to detect when a popup opens. This does not work:</p>
  
  <pre><code>let popup = new mapboxgl.Popup()
  popup.on('open', function(e) {
      alert()
  })
  </code></pre>
  
  <p><strong>EDIT:</strong></p>
  
  <p>The popup event is now supported, so the example above should work.</p>