HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>error UI</title>
  <style>
    #game { width: 320px; height: 160px; border: 2px solid #333; background: #eee; padding: 12px; }
    #game.is-fallback { background: #1a1a2e; color: #fff; }
  </style>
</head>
<body>
  <div id="game">
    <p>Game surface</p>
    <button type="button" id="go">Lock pointer</button>
  </div>
  <pre id="msg">(waiting)</pre>
  <script>
    var el = document.getElementById("game");
    var msg = document.getElementById("msg");
    document.addEventListener("pointerlockerror", function () {
      msg.textContent = "Pointer lock blocked here. Using mouse-move aiming without lock.";
      el.classList.add("is-fallback");
    });
    document.addEventListener("pointerlockchange", function () {
      if (document.pointerLockElement) {
        msg.textContent = "Pointer lock OK";
        el.classList.remove("is-fallback");
      }
    });
    document.getElementById("go").addEventListener("click", function () {
      el.requestPointerLock();
    });
  </script>
</body>
</html>

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Document pointerlockerror — friendly UI fallback (Try It)