body::before {
  content: "";
  position: fixed;
  top: -10px;
  left: 0;
  width: 100%;
  height: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.8);
  z-index: 100;
}

利用伪元素实现，并设置 position:fixed;