Show hidden v-show exception in [bug report] menu

### Element UI version
2.15.0

### OS/Browsers version
win7 chrome

### Vue version
2.6.12

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

### Steps to reproduce
通過v-show控制menu顯示隱藏時
執行兩次后menu寬度發生變化了
用v-if控制時,隱藏前會閃現以上問題

### What is Expected?
多次顯示隱藏樣式應該一致

### What is actually happening?
多次顯示隱藏樣式不一致