[Bug Report] El-submenu causes 'Maximum call stack exceeded' error in collapsed menu if it's the top element of a component

### Element UI version
2.12.0

### OS/Browsers version
Windows 10 OS Version 1809 (Build 17763.805) / 78.0.3904.70 (Official Build) (64-bit) (cohort: 78_70_Win)

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/mik-jozef-artin/pen/abbYNza

### Steps to reproduce
0. Open the [reproduction link](https://codepen.io/mik-jozef-artin/pen/abbYNza).
1. Place mouse on the menu (ie. the text 'title')

The error 'Maximum call stack size exceeded' will appear in the console.

It seems to me that these conditions are all necessary for the issue to manifest:

0. Menu must be collapsed (the `collapse` prop on `el-menu` must be `true`)
1. Submenu must be in a separate component (in this case `customized-menu-item`)
2. The submenu must be the top element in the separate component.
3. Attribute `:popper-append-to-body="true"` must be present on the submenu

Fortunately, the second condition provides an **easy workaround** - placing the submenu inside a `div`
will suppress the issue. This behavior is still surprising and I guess unwanted.

### What is Expected?
No errors.

### What is actually happening?
`Uncaught RangeError: Maximum call stack size exceeded.`

Start with the linked CodePen reproduction and inspect the collapsed el-menu containing the top-level el-submenu, especially with popper-append-to-body enabled. Trace the hover path that produces the maximum call stack error and compare it with the div-wrapped workaround. Done means the reproduction no longer logs the error while preserving the collapsed submenu behavior.

Written by the indexing model from the issue text.