before_unload_prevent_default.html | Explore in Territory

<!DOCTYPE html>
<html>
  <script>
    window.addEventListener('beforeunload', function (e) {
      e.preventDefault();
      e.returnValue = '';
    });
  </script>
  <body>
    <p>Hello, world!</p>
  </body>
</html>

chromium/chrome/browser/headless/test/data/before_unload_prevent_default.html