[bug report] some browsers of computers use drop-down boxes and cascade selectors to select multiple items. As the level of drop-down box positioning increases, the page jitters frequently.

### Element UI version
2.14.1

### OS/Browsers version
win10 / google

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/pen

### Steps to reproduce
1.

### What is Expected?
页面不抖动

### What is actually happening?
页面因为层级的不断增加的原因，频繁抖动