lazyload-contents.html

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<!DOCTYPE html>

<body>

  <p>This page is lazyloaded.</p>

  <script>

    let query_index = window.location.href.indexOf("?id=");

    let id = window.location.href.substr(query_index + 4);

    window.addEventListener("load", () => {

      let p = document.querySelector("p");

      let contents = p ? p.innerHTML : "null";

      window.parent.postMessage({"id": id, "contents": contents}, "*");

});

  </script>

</body>