[Bug Report] 表格固定列，滑动横向滚动出现内容样式显示异常

[zhangbjing]

Element UI version

2.15.14

OS/Browsers version

chrome

Vue version

2.6.12

Reproduction Link

https://private-user-images.githubusercontent.com/32285384/523624375-5a0fd28e-c84b-46df-859f-728505cebcb0.png?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.LU6VviBrDDmd_zXg0ulBAOBdCx_kNWyhS9Rb5wsyw4U

Steps to reproduce

表格右侧存在固定列，来回滑动表格横向滚动条和竖向滚动条会出现本该隐藏在固定列下方的内容显示在固定列

What is Expected?

表格内容显示正常，固定列下方内容不会穿透显示

What is actually happening?

固定列下方的内容穿透在固定列上方显示了

[wutong21]

我们在项目中好像也遇到了相同的问题，似乎是因为.el-table-fixed 没有设置背景色，如果背景色是白色，可以给.el-table__fixed, .el-table__fixed-right 设置个background-color: #fff;