[bug report] fixed total row at the end of the list. If there are too many column names, the scroll bar cannot be selected in the fixed area (cannot scroll horizontally)

### Element UI version
2.15.5

### OS/Browsers version
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.131 Safari/537.36

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/ghost-001/pen/NWjZxmQ

### Steps to reproduce
第一个表格，无法在固定列区域拖动滚动条

### What is Expected?
可以拖动

### What is actually happening?
无法拖动