<!DOCTYPE html>
  <html>
    <head>
      <link id="prefetch" rel="prefetch" href="pass.png">
    </head>
    <body>
      <script>
        const bc = new BroadcastChannel(new URLSearchParams(location.search).get("uid"));
        document.getElementById("prefetch").addEventListener("error", e => bc.postMessage(false));
        const observer = new PerformanceObserver(entries => {
          const found = entries.getEntriesByName(new URL("pass.png", location.href).href);
          if (found.length)
            bc.postMessage(found[0].encodedBodySize > 0);
        });
        observer.observe({entryTypes: ["resource"]});
      </script>
    </body>
  </html>
