[Bug Report] When drag is turned on, sometimes the border and area of ​​the dragged file content flashes, hope to fix it

### Element UI version
2.13.2

### OS/Browsers version
windows 10

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/pen/?&editable=true=https%3A%2F%2Felement.eleme.io%2F

### Steps to reproduce
Quickly drag files into the drag area or drag files in the drag area

### What is Expected?

Display the drag-in style normally

### What is actually happening?
Keep flashing