[bug report] when a horizontal scroll bar appears on the page and the position of the El table needs to be scrolled horizontally to be displayed, the whole header of the table becomes draggable.

### Element UI version
2.15.6

### OS/Browsers version
macos 11.4 / chrome 95.0.4638.54（正式版本） (x86_64)

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/yemubing/pen/LYjzmqK

### Steps to reproduce
见 重现链接

### What is Expected?
需要滚动才能显示的el-table，只是在表头的单元格的边框附近才能拖拽。

### What is actually happening?
需要滚动才能显示的el-table，整个表头都能拖拽。