如何利用CSS的border-radius和height属性实现动态阴影效果？

<div class="box">
  <div class="mask"></div>
</div>

.box {
  width: 200px;
  height: 200px;
  position: relative;
}

.box .mask {
  top: 0;
  left: 0;
  right: 0;
  position: absolute;
  background-color: rgba(0, 0, 0, 0.3); /* 半透明黑色背景 */
}

.box1 .mask { /* 例子：可调整参数 */
  height: 50px;
  border-radius: 0 0 100% 100%;
}

const box = document.getElementsByClassName('box3')[0]; // 选择可拖动的元素
let y = 0;

box.addEventListener('mousedown', ({ offsetY }) => {
  box.movable = true;
  y = offsetY;
});

box.addEventListener('mouseup', () => {
  box.movable = false;
  y = 0;
});

box.addEventListener('mousemove', ({ offsetX, offsetY }) => {
  if (box.movable) {
    const w = parseFloat(getComputedStyle(box).width);
    setLRH(
      Math.round(offsetX / w * 100),
      Math.round(100 - offsetX / w * 100),
      offsetY - y
    );
  }
});

function setLRH(l, r, h) {
  const mask = box.getElementsByClassName('mask')[0];
  mask.style.borderRadius = `0 0 ${r}% ${l}%`;
  mask.style.height = `${h}px`;
}