在真机可以看到有很明显的锯齿

<view class="cover"></view>

.cover {
  position: relative;
  overflow: hidden;
  width: 100%;
  height: 24px;
  background: black;


  &:after {
    content: '';
    position: absolute;
    width: 400vw;
    height: 400vw;
    left: -150%;
    border-radius: 50%;
    background: red;
  }
}