<!DOCTYPE html>
<html>
<head>
<script>

function boom()
{
  var s = "x";
  for (var i = 0; i < 15; ++i)
    s = s + s;
  var t = document.createTextNode(s);
  document.body.appendChild(t);
  window.getSelection().collapse(t, s.length);
  document.execCommand("insertText", false, "a");
}

</script>
</head>

<body contenteditable="true" onload="boom();"></body>
</html>