That's because your component extends from a different root "Vue" prototype than the "localVue" you created. There's little this lib can do "in the background" to prevent this, I think.

A quick solution would be to run your component through localVue's extend() again:

const localVue = createLocalVue()
localVue.component('GloballyLoaded', GloballyLoaded)
const component = localVue.extend(App)

describe('HelloWorld.vue', () => {
  it('renders props.msg when passed', async () => {
    const wrapper = mount(component, {
      localVue
    })
    await sleep(2000)
    console.log(wrapper.html())
    expect(true).toBeTruthy()
    // expect(wrapper.text()).toMatch(msg)
  })
})

I don't understand why this should be a thing because the error only applies if the component is loaded async. If it is loaded sync everything is working fine

But if tried your idea and unfortunately it doesn't fix the issue :(

It looks this is an outstanding issue that has never been supported: #1012

Should this issue be renamed "Test fails for asynchronously loaded components"? Seems like it's related to that, not TS/Vue Class Component (could be wrong, I didn't pull your repo yet).

We should definitely support async components this, I'm not sure what (if anything) makes it difficult to implement. Is there some way for Jest to know about Webpack chunks? In fact, you will need to do a full build of the app prior to running the tests for the chunks to even exist? (not super familiar with the webpack code splitting stuff...)

Well async loaded component are working fine as long as you don't use vue.extend.
But vue.extend is used by the typescript library for vue. So a typescript vue components can't be loaded async. Because of that i found this issue.

I'm pretty suprised that this feature isn't really implemented but is working fine for normale vue files :D