chore: add some comments

Author: lmiller1990

src/mount.ts

@@ -425,12 +425,20 @@ export function mount(
 
   // mount the app!
   const vm = app.mount(el)
-  const appRef = vm.$refs[MOUNT_COMPONENT_REF] as ComponentPublicInstance
 
+  // Ingore Avoid app logic that relies on enumerating keys on a component instance... warning
   const warnSave = console.warn
   console.warn = () => {}
+
+  // get `vm`.
+  // for some unknown reason, getting the `vm` for components using `<script setup>`
+  // as of Vue 3.0.3 works differently.
+  // if `appRef` has keys, use that (vm always has keys like $el, $props etc).
+  // if not, use the return value from app.mount.
+  const appRef = vm.$refs[MOUNT_COMPONENT_REF] as ComponentPublicInstance
   const $vm = Reflect.ownKeys(appRef).length ? appRef : vm
   console.warn = warnSave
+
   return createWrapper(app, $vm, setProps)
 }
 

tests/components/ScriptSetup.vue

@@ -1,6 +1,7 @@
 <script setup lang="ts">
 // imported components are also directly usable in template
 import { ref } from 'vue'
+import Hello from './Hello.vue'
 
 // write Composition API code just like in a normal setup()
 // but no need to manually return everything
@@ -10,4 +11,5 @@ const inc = () => { count.value++ }
 
 <template>
   <button @click="inc">{{ count }}</button>
+  <Hello />
 </template>

tests/features/sfc.spec.ts

@@ -11,6 +11,8 @@ describe('sfc', () => {
   // rfc: https://github.com/vuejs/rfcs/pull/228
   it('works with <script setup> (as of Vue 3.0.3)', async () => {
     const wrapper = mount(ScriptSetup)
+    expect(wrapper.findComponent(Hello).exists()).toBe(true)
+
     await wrapper.find('button').trigger('click')
     expect(wrapper.html()).toContain('1')
   })