现象

el-input被外层div包裹了，设置autofocus不起作用：

<el-dialog v-model="visible" :title="title" :append-to-body=true width="50%">
    <el-form v-model="theForm" @submit.prevent="submit">
      <el-form-item v-for="item in formItems" :label="item.label" :label-width="120">
        <el-input v-model="theForm[item.name]" ref="inputRef"
          autocomplete="off"           
          @keyup.enter.stop="submit"
          autofocus/>
      </el-form-item>
    </el-form>
    <template #footer>
        <span>
          <el-button type="primary" @click="cancel()">取消</el-button>
          <el-button type="primary" @click="submit()">确定</el-button>
        </span>
    </template>
  </el-dialog>

在onMounted()中尝试: inputRef.value.focus()，也无效。怀疑是调用时机不对，但何时才是页面dom都生成好的时机呢？

两次nextTick()加setTimeout()解决

// 需要调用两次nextTick才能获得实例!!!
const focusInput = () => { nextTick(() => 
  nextTick(()=>{    
    setTimeout(function () {
      let _eles = document.querySelectorAll('form input.el-input__inner')
      // console.log(_eles)
      _eles[0].focus()
      // inputRef.value.focus()
    }, 100);
    
  }))}

onMounted(() => {
  focusInput()
})

结论

在vue3组件嵌套复杂时，很难确定组件之间的初始化顺序，这时尽可能让访问时机靠后是一个通用的解决方法。