[bug report] navmenu side menu can't scroll when the number of secondary menus is too much when it is folded and exceeds the screen range

### Element UI version
2.13.0

### OS/Browsers version
window/mac chrome 80.0.3987.132

### Vue version
2.5.17

### Reproduction Link
https://codepen.io/13522119901/pen/abORmNY?editable=true

### Steps to reproduce
侧边菜单收起状态下显示二级菜单会显示不全 无法滚动看到所有的二级菜单

### What is Expected?
屏幕较小、二级菜单数量过多时可以滚动显示

### What is actually happening?
屏幕较小、二级菜单数量过多时部分菜单无法显示

Start with the linked CodePen reproduction and inspect the nav menu's folded state and secondary-menu rendering. Reproduce the issue in a small viewport with many secondary menus, then trace the relevant frontend component behavior. Done means every secondary menu can be reached by scrolling when the menu is folded.

Written by the indexing model from the issue text.