pointer-events文档
解决思路为通过pointer-events实现事件穿透，不响应固定列的拖拽，而是响应其子元素的拖拽事件

/deep/.el-table__fixed,
/deep/.el-table__fixed-right {
  pointer-events: none;
}
/deep/.el-table__fixed *,
/deep/.el-table__fixed-right * {
  pointer-events: auto;
}