Click the date-picker inside the popover (the click time is extended appropriately). Popovers disappear when the display height is insufficient

### Element UI version
2.13.0

### OS/Browsers version
macOS 10.13.6/ Chrome 78.0.3904.108

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/381510688/k4scm5u8/7/

### Steps to reproduce
1. 缩小展示区域
2. 点击open
3. popover可以出现，点击日历控件（click时间适当加长一些）
4. popover消失了

### What is Expected?
popover、date-picker 都显示正常

### What is actually happening?
popover 消失了，date-picker 显示正常