HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>moveBefore focus</title>
  <style>
    .box { padding: 12px; margin: 8px; border: 2px solid #333; min-height: 40px; }
    #boxA { background: #e3f2fd; }
    #boxB { background: #fff3e0; }
  </style>
</head>
<body>
  <div class="box" id="boxA"><input id="field" value="Type here"></div>
  <div class="box" id="boxB">Target box</div>
  <button type="button" id="btn">Move focused input</button>
  <pre id="out">Focus the input, then click move</pre>
  <script>
    var field = document.getElementById("field");
    var boxB = document.getElementById("boxB");
    function moveNode(parent, node, ref) {
      if (typeof parent.moveBefore === "function") parent.moveBefore(node, ref);
      else parent.insertBefore(node, ref);
    }
    document.getElementById("btn").onclick = function () {
      field.focus();
      var hadFocus = document.activeElement === field;
      moveNode(boxB, field, null);
      document.getElementById("out").textContent = "Focus preserved: " + (hadFocus && document.activeElement === field);
    };
  </script>
</body>
</html>

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Element.moveBefore — focus preserved (Try It)