[Bug Report] If there is a full screen effect on a DIV, the Popover and Tooltip will not display properly.如果一个DIV开启全屏效果，则Popover和Tooltip不能正常显示。

### Element UI version
2.12.0

### OS/Browsers version
Windows 10 Pro 1903 / Google Chrome 77.0.3865.90

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/3ej0x5yf/

### Steps to reproduce
click the blue button to full screen, the Popover and Tooltip will not display properly

### What is Expected?
If there is a full screen effect on a DIV, the Popover and Tooltip is ok.

### What is actually happening?
If there is a full screen effect on a DIV, the Popover and Tooltip will not display properly