Active (focused) editor loses focus on updateContents() for another editor in one page

[cutteroid]

If multiple editors are on page, when one editor focused and another one receives content updates with formatting the first editor loses focus.

Steps for Reproduction

1. Visit http://beta.quilljs.com/playground/

2. Insert HTML code

   <div id="editor-container1">
       <p><br></p>
   </div>
   
   <div id="editor-container2">
       <p><br></p>
   </div>

3. Insert JS code

   var quill1 = new Quill('#editor-container1', {
       placeholder: 'Compose an epic...', theme: 'snow'
   });
   
   var quill2 = new Quill('#editor-container2', {
       placeholder: 'Compose an epic...', theme: 'snow'
   });
   
   quill1.on('text-change', function(delta, oldDelta, source) {
       if ( source == 'user' ) { quill2.updateContents(delta);  }
   });

4. Select some format (for example, bold) in top editor and start typing

Expected behavior: bold symbols appearing in both editors

Actual behavior: focused editor loses focus after one bold symbol entered

Platforms: Firefox 47, Firefox 49.0a2 and Chrome 51.0.2704.106 on OS X

Version: 1.0.0-beta.9

This misbehavior appeared after 7f1fa0d commit