如何给scrollLeft变化的元素添加动画效果呢？

给滚动容器设置 scroll-behavior 属性做平滑滚动？

<div id="container">
  <div id="content">Click the button to slide right!</div>
</div>

<button id="slide" type="button">Slide right</button>

#container {
  width: 100px;
  height: 100px;
  border: 1px solid #ccc;
  overflow-x: scroll;
  scroll-behavior: smooth;
}

#content {
  width: 250px;
  background-color: #ccc;
}

const button = document.getElementById("slide");

button.onclick = function () {
  document.getElementById("container").scrollLeft += 20;
};