HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>startViewTransition return</title>
  <style>
    body { font-family: sans-serif; margin: 1rem; }
    #panel { padding: 1rem; background: #fef3c7; border-radius: 8px; }
    #panel[hidden] { display: none; }
    button { margin-top: 0.5rem; }
    pre { font-family: monospace; }
  </style>
</head>
<body>
  <div id="panel">Panel content</div>
  <button type="button" id="toggle">Toggle panel</button>
  <pre id="out">—</pre>
  <script>
    var panel = document.getElementById("panel");
    document.getElementById("toggle").addEventListener("click", function () {
      function update() { panel.hidden = !panel.hidden; }
      if (!panel.startViewTransition) { update(); document.getElementById("out").textContent = "No API — instant toggle"; return; }
      var vt = panel.startViewTransition(update);
      document.getElementById("out").textContent = "Transition started…";
      vt.finished.then(function () {
        document.getElementById("out").textContent = "element transition finished";
      });
    });
  </script>
</body>
</html>

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Element.startViewTransition — ViewTransition return (Try It)