[Bug Report] When el-table use show-summary attr. and the columns is fixed ,scroll bar not work

### Element UI version
2.15.14

### OS/Browsers version
chrome

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/varsha-xiu/pen/MYjXKNW

### Steps to reproduce
When the first and last columns are fixed, and the horizontal scrollbar is scrolled below the fixed columns, the scrollbar cannot be clicked, meaning it cannot be scrolled.

### What is Expected?
When the scrollbar is below the fixed column, it can be scrolled

### What is actually happening?
When the scrollbar is below the fixed column, it can not be scrolled