如何在颤动中弹出没有动画的屏幕

<button onclick="showPopup()">弹出屏幕</button>
<div id="popup" class="popup">
  <div class="content">
    <h1>弹出屏幕内容</h1>
    <p>这是一个没有动画的弹出屏幕。</p>
  </div>
</div>

.popup {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 300px;
  height: 200px;
  background-color: #fff;
  border: 1px solid #000;
  display: none;
}

.popup.show {
  display: block;
}

.content {
  padding: 20px;
}

button {
  margin-top: 20px;
}

function showPopup() {
  var popup = document.getElementById("popup");
  popup.classList.add("show");
  
  setTimeout(function() {
    popup.classList.remove("show");
  }, 1000);
}