<!DOCTYPE html>
<body>
  <script>
    function handleMessage(e) {
      if (e.data == "focus") {
        let button = document.querySelector("button");
        button.onfocus = () => e.source.postMessage({focused: true}, {targetOrigin: "*"});
        button.focus();
      }

      if (e.data == "focus-update-active-element") {
        let button = document.querySelector("button");
        button.focus();
        e.source.postMessage({activeElementUpdated: document.activeElement === button}, {targetOrigin: "*"});
      }
    }

    if (window.portalHost)
      window.portalHost.onmessage = handleMessage;

    window.onmessage = handleMessage;

    window.onportalactivate = e => {
      let portal = e.adoptPredecessor();
      document.body.appendChild(portal);
      portal.onmessage = handleMessage;
    };

    window.onfocus = () => {
      if (window.portalHost)
        window.portalHost.postMessage("window focused");
    };
  </script>
  <button>A</button>
</body>
