[Bug Report] InputNumber control buttons do not always work

https://user-images.githubusercontent.com/7934525/105369676-c87ea400-5bd0-11eb-9d47-3494d005f95f.mp4

### Element UI version
2.15.0

### OS/Browsers version
Windows 10/Chrome Latest

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/t012au5z/5/

### Steps to reproduce
Click the step button for the InputNumber

### What is Expected?
The value to increase by one each time.

### What is actually happening?
Even though the click is registered, sometimes the value does not increase.