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<!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>