Popper: Invalid window object when detached to a new window

### Element UI version
2.13.0

### OS/Browsers version
Linux / Google Chrome / 79.0.3945.88 (Official Build) (64-bit)

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/1z39ayLb/15/

### Steps to reproduce
Click on the 'open' button, look inside the javascript console for the error.

### What is Expected?
The el-cascader should use the element.ownerDocument.defaultView as window object to query the computed style.

### What is actually happening?
It uses a global root object initialized to the global window object thus the DOM element doesn't belong to the appropriate window and it doesn't work.