<div class="tree">  
  <el-tree>
  </el-tree>  
</div>

.tree {
  overflow-y: auto;  
  overflow-x: scroll;
  width: 80px;
  height: 500px;  
  background-color: #ffffff;  
}

.el-tree {  
  min-width: 100%;
  display: inline-block !important;  
}

.el-tree-node > .el-tree-node__children {
  overflow: visible;