Element-UI 的 折疊Menu的相關(guān)CSS過(guò)渡是怎么來(lái)的？

<el-radio-group v-model="isCollapse" style="margin-bottom: 20px;">
  <el-radio-button :label="false">展開</el-radio-button>
  <el-radio-button :label="true">收起</el-radio-button>
</el-radio-group>
<el-menu default-active="1-4-1" class="el-menu-vertical-demo" @open="handleOpen" @close="handleClose" :collapse="isCollapse">
  <el-submenu index="1">
    <template slot="title">
      <i class="el-icon-location"></i>
      <span slot="title">導(dǎo)航一</span>
    </template>
    <el-menu-item-group>
      <span slot="title">分組一</span>
      <el-menu-item index="1-1">選項(xiàng)1</el-menu-item>
      <el-menu-item index="1-2">選項(xiàng)2</el-menu-item>
    </el-menu-item-group>
    <el-menu-item-group title="分組2">
      <el-menu-item index="1-3">選項(xiàng)3</el-menu-item>
    </el-menu-item-group>
    <el-submenu index="1-4">
      <span slot="title">選項(xiàng)4</span>
      <el-menu-item index="1-4-1">選項(xiàng)1</el-menu-item>
    </el-submenu>
  </el-submenu>
  <el-menu-item index="2">
    <i class="el-icon-menu"></i>
    <span slot="title">導(dǎo)航二</span>
  </el-menu-item>
  <el-menu-item index="3">
    <i class="el-icon-setting"></i>
    <span slot="title">導(dǎo)航三</span>
  </el-menu-item>
</el-menu>

<style>
  .el-menu-vertical-demo:not(.el-menu--collapse) {
    width: 200px;
    min-height: 400px;
  }
</style>

<script>
  export default {
    data() {
      return {
        isCollapse: true
      };
    },
    methods: {
      handleOpen(key, keyPath) {
        console.log(key, keyPath);
      },
      handleClose(key, keyPath) {
        console.log(key, keyPath);
      }
    }
  }
</script>

看了Element-UI的源碼，不是很了解展開和收起的相關(guān)過(guò)渡是怎么完成的。