HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>fullscreen button guard</title>
  <style>
    #stage { padding: 1rem; background: #0f172a; color: #f8fafc; }
  </style>
</head>
<body>
  <div id="stage">Stage</div>
  <p><button id="fs-btn" type="button">Enter fullscreen</button></p>
  <pre id="out"></pre>
  <script>
    var btn = document.getElementById("fs-btn");
    var out = document.getElementById("out");
    if (!document.fullscreenEnabled) {
      btn.disabled = true;
      btn.title = "Fullscreen is not available in this context";
      out.textContent = "Button disabled — fullscreenEnabled is false";
    } else {
      btn.addEventListener("click", function () {
        document.getElementById("stage").requestFullscreen().catch(function (e) {
          out.textContent = String(e);
        });
      });
      out.textContent = "Button enabled — fullscreenEnabled is true";
    }
  </script>
</body>
</html>

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