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<!DOCTYPE html>
<html>
<body>
<button id="first">First</button>
<button id="last">Last</button>
<script>
window.addEventListener("focusin", e => {
const isWindow = e.target == window;
const name = isWindow ? "window" : e.target.tagName;
window.parent.postMessage({ id: e.target.id, name }, "*");
});
</script>
</body>
</html>