title: CodeMirror editor scrolls when typing in Firefox
body: <p>I have an issue with CodeMirror editor on Firefox. When I type something in the bottom line, every first symbol in scrolls top (making current line invisible) and every second symbol it scrolls back.</p>
  
  <p>Issue occurs only in Firefix 34 on OSX 10.10.1, but not in Chrome or Safari.</p>
  
  <p>Noting unusual:</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;textarea id="txt"&gt;Hello my dear friend!
  Hello my dear friend!
  Hello my dear friend!
  Hello my dear friend!
  Hello my dear friend!
  Hello my dear friend!
  Hello my dear friend!&lt;/textarea&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>var editor = CodeMirror.fromTextArea(document.getElementById("txt"), {
      mode: 'xml',
      lineWrapping: true
  });
  </code></pre>
  
  <p>CSS:</p>
  
  <pre><code>.CodeMirror {
      width: 200px;
      height: 100px;
      border: 1px solid #ccc;
  }
  </code></pre>
  
  <p>Here is a small <a href="http://jsfiddle.net/6oqu5asz/" rel="nofollow">JSFiddle</a>. Just put cursor after the last symbol, press Enter and start typing.</p>