[bug report] when El table uses expanded row, when a column of the outer table uses fixed = "right", a row is expanded, and the hover events of the outer table are disordered

### Element UI version
2.15.3

### OS/Browsers version
macos big sur 11.2.3 / chrome 91.0.4472.114（正式版本） (x86_64)

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/shaolongwen/pen/PomZwMd

### Steps to reproduce
展开外层table的第一行数据后，鼠标移入外层table的第二行数据

### What is Expected?
变更鼠标当前移入的数据的背景色

### What is actually happening?
变更了展开第一行数据里的第一行数据的背景色