如何在子窗口顶部显示弹出窗口

<div id="popupContainer"></div>

// 监听某个按钮的点击事件
document.getElementById('openPopupButton').addEventListener('click', function() {
  // 创建弹出窗口的内容
  var popupContent = document.createElement('div');
  popupContent.innerHTML = '这是弹出窗口的内容';

  // 将弹出窗口的内容添加到容器元素中
  var popupContainer = document.getElementById('popupContainer');
  popupContainer.appendChild(popupContent);

  // 设置弹出窗口的样式
  popupContainer.style.position = 'fixed';
  popupContainer.style.top = '0';
  popupContainer.style.left = '0';
  popupContainer.style.width = '100%';
  popupContainer.style.height = '50px';
  popupContainer.style.backgroundColor = '#f1f1f1';
  popupContainer.style.zIndex = '9999';
});