LayoutTests/http/tests/site-isolation/selection-focus.html

<!-- webkit-test-runner [ SiteIsolationEnabled=true ] -->
<script>
    function setFocus() {
        frame.focus();
    }

    addEventListener("message", (event) => {
        document.body.focus();
    });
</script>
<iframe onload="setFocus()" id="frame" src="http://localhost:8000/site-isolation/resources/selected-text.html"></iframe>