backTop组件的target指向的对象没有缓动效果

[daskyrk]

What problem does this feature solve?

项目内顶部固定，滚动的是一个指定的div元素，通过target指定该滚动元素后，由于scrollToTop方法中，currentScrollTop()获得的总是window或body的scrollTop，所以缓动函数总是返回0，导致指定的滚动元素立即回到顶部。
既然有这样一个外部接口提供target，或许对target也进行动画处理比较好。

What does the proposed API look like?

currentScrollTop(this.props.target());
// 引入target
const currentScrollTop = (target) => {
  const targetScrollTop = target && target.scrollTop;
  return targetScrollTop || window.pageYOffset || document.body.scrollTop || document.documentElement.scrollTop;
};

[afc163]

Did you propose a bug but a feature request?

[afc163]

I think this is a bug, you should choose bug report in issue templates.

[lock]

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