[bug report] the border jumps when the collapse component is folded

### Element UI version
2.13.2

### OS/Browsers version
window10/Chrome79.0.3945.88

### Vue version
3.0.0-beta.20

### Reproduction Link
https://codepen.io/pen/?&editable=true=https%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce
存在两个CollapseItem及以上时，折叠CollapseItem面板

### What is Expected?
折叠Collapse面板时上下边框重合瞬间无跳动现象

### What is actually happening?
折叠CollapseItem面板时,上面的一个CollapseItem会先产生下边框，折叠完成瞬间会有一个此处边框宽度增大的错觉