view/test.html

<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>666666</title>
  <link rel="stylesheet" href="https://cdn.bootcss.com/twitter-bootstrap/4.1.2/css/bootstrap.min.css" />
  <script src="https://cdn.bootcss.com/jquery/3.3.0/jquery.min.js"></script>
  <script src="https://cdn.bootcss.com/twitter-bootstrap/4.1.2/js/bootstrap.min.js"></script>
  <style>
    textarea.sansserif{font-family:Arial,Helvetica,sans-serif; font-size:100%;}
    textarea.ex {color:rgb(0,0,255);}
    textarea.wh {width:100%;}
  </style>
</head>

<body onload="init();">

  <div>
    <textarea class="sansserif ex wh" id="codePanel">
      123
      123
      444
      555
      666
    </textarea>
  </div>
  <script type="text/javascript">
    var observe;
    if (window.attachEvent) {
      observe = function (element, event, handler) {
        element.attachEvent('on' + event, handler);
      };
    } else {
      observe = function (element, event, handler) {
        element.addEventListener(event, handler, false);
      };
    }

    function init() {
      var text = document.getElementById('codePanel');

      function resize() {
        text.style.height = 'auto';
        text.style.height = text.scrollHeight + 'px';
      }
      /* 0-timeout to get the already changed text */
      function delayedResize() {
        window.setTimeout(resize, 0);
      }
      observe(text, 'change', resize);
      observe(text, 'cut', delayedResize);
      observe(text, 'paste', delayedResize);
      observe(text, 'drop', delayedResize);
      observe(text, 'keydown', delayedResize);
      text.focus();
      text.select();
      resize();
    }
  </script>
</body>

</html>