[bug report] too many calculations in the cascaded form cause the scrolling to get stuck

[AzronChan]

Element UI version

2.15.5

OS/Browsers version

window10/node 14

Vue version

2.6.14

Reproduction Link

https://codepen.io/pen/?&prefill_data_id=78731dc4-107d-4c29-9ef3-6286d71da51f

Steps to reproduce

级联元素插入到body时，就开始监听页面滚动，设置位置，导致页面卡顿

What is Expected?

级联不展示时，不应该计算级联位置

What is actually happening?

级联不展示时，计算级联位置，导致页面滚动卡顿

[element-bot]

Translation of this issue:

Element UI version

2.15.5

OS/Browsers version

window10/node 14

Vue version

2.6.14

Reproduction Link

https://codepen.io/pen/?& prefill_ data_ id=78731dc4-107d-4c29-9ef3-6286d71da51f

Steps to reproduce

When the cascading element is inserted into the body, it starts to listen to page scrolling and set the position, resulting in page jamming

What is Expected?

The cascade position should not be calculated when the cascade is not displayed

What is actually happening?

When the cascade is not displayed, the cascade position is calculated, resulting in page scrolling and jamming

<img src=" https://ruqi-img.ruqimobility.com/pic_ image/20210812063814__ 9SJH16OH.gif" />

[XKF]

Nice！用的旧版本刚想来提这个issue，然后发现官网最新版解决了