聚焦iframe时,不调用父窗口事件。如何预防?

document.onkeydown = e => {
  e.preventDefault();
  console.log(e.key);
}

onblur = e => {
  if (e.currentTarget === e.target) {
    focus(); // FF
    setTimeout(focus, 20); // Chrome
  }
};
overlay.onclick = function() {
  this.remove()
};

#overlay {
  position: absolute;
  text-align: center;
  background: white;
  width: 100vw;
  height: 100vh;
}

<div id="overlay"><button>begin demo</button></div>
<iframe srcdoc="<input value='can not type here'><button onclick='alert(`clicked`)'>button still works</button>"></iframe>