<script>
window.onload = () => {
  window[0].focus()
  a.close()
  a.showModal()
  document.execCommand("selectAll", false)
}
function go() {
  document.onselectstart = document.createElement("frameset").onload
}
</script>
<details open="true" ontoggle="go()">a</details>
<iframe></iframe>
<dialog id="a" tabindex="3">a</dialog>
