#topBtn {
  position: fixed;
  right: 20px;
  bottom: 20px;
  opacity: 0;
  visibility: hidden;
  transition: 0.3s;
  border: none;
  background: none;
  padding: 0;
  cursor: pointer;
  /* 关键：去除点击/聚焦时的边框 */
  outline: none;
}

#topBtn.show {
  opacity: 1;
  visibility: visible;
}

#topBtn img {
  width: 80px; /* 图标宽度 */
  height: auto;
  /* 防止图片本身可能出现的边框 */
  border: none;
  outline: none;
}

html {
  scroll-behavior: smooth;
}