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<!DOCTYPE html>
<body>
<button id="one">one</button>
<button id="two" autofocus>two</button>
<button id="three">three</button>
<script>
function messageHandler(e) {
if (e.data === 'check-active-element') {
window.requestAnimationFrame(() => {
let autofocusedButton = document.querySelector('#two');
e.source.postMessage(document.activeElement === autofocusedButton);
});
}
}
window.portalHost.onmessage = messageHandler;
window.onportalactivate = e => {
let portal = e.adoptPredecessor();
portal.onmessage = messageHandler;
document.body.appendChild(portal);
}
</script>
</body>