[Bug Report] With fixed columns, scroll bars cannot be dragged， Scroll bar for dragging fixed column part cannot be dragged, Important: add fixed column, add total, and then drag the horizontal scroll bar of fixed column

### Element UI version
2.13.0

### OS/Browsers version
win

### Vue version
3.0.0-alpha.1

### Reproduction Link
https://codepen.io/livelypeng/pen/NWPwLeo

### Steps to reproduce
Important: add fixed column, add total, and then drag the horizontal scroll bar of fixed column
1. Add fixed column
2. Add in total
3. Scroll bar for dragging fixed column part cannot be dragged

Important: add fixed column, add total, and then drag the horizontal scroll bar of fixed column

### What is Expected?
You can drag the scroll bar of the fixed column section

### What is actually happening?
Do not drag