[Bug Report] Cascader component causes Popper to increase zindex at 110% of the page

### Element UI version
2.12.0

### OS/Browsers version
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.75 Safari/537.36

### Vue version
2.6.10

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

### Steps to reproduce
点击input，显示popper，打开控制台，可以看到元素class="el-popper el-cascader__dropdown"到zindex一直在递增抖动

### What is Expected?
不发生抖动，zindex不发生递增

### What is actually happening?
发生抖动，zindex发生递增