vue jest单元测试中 element-ui 的 el-dropdown组件 模拟 click 后，视图为什么无法更新

Question

test组件

    <el-dropdown>
        <el-button>test</el-button>
        <el-dropdown-menu slot="dropdown">
            <el-dropdown-item command="add">增加</el-dropdown-item>
            <el-dropdown-item command="template">mo</el-dropdown-item>
        </el-dropdown-menu>
    </el-dropdown>

test.spec.js

import { createLocalVue, mount } from '@vue/test-utils'
import test from '@/components/test.vue'
import ElementUI from 'element-ui'
const localVue = createLocalVue();
localVue.use(ElementUI);

const transitionStub = () => ({
  render: function (h) {
    return this.$options._renderChildren
  }
})

describe('test.vue', () => {
  const wrapper = mount(test, {
    stubs: {
      transition: transitionStub()
    },
    localVue
  })
  it('test', async () => {
    await wrapper.find('button').trigger('click');
    expect(wrapper.find('ul').isVisible()).toBe(true);
  })
})

为什么这个断言无法通过？