Duplicate root app (id="app") when creating vue3 project

[kirianguiller]

Version

4.5.4

Reproduction link

https://github.com/kirianguiller/vue3-issue

Environment info

  System:
    OS: Linux 5.4 Ubuntu 20.04 LTS (Focal Fossa)
    CPU: (8) x64 Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
  Binaries:
    Node: 10.19.0 - /usr/bin/node
    Yarn: Not Found
    npm: 6.14.7 - /usr/local/bin/npm
  Browsers:
    Chrome: Not Found
    Firefox: 79.0
  npmGlobalPackages:
    @vue/cli: 4.5.4

Steps to reproduce

• Create a vue3 app with vue CLI 4.5.4
• Run 'npm run serve'

What is expected?

Only one "id='app'" html element

What is actually happening?

Two "id='app'" html element

---

These duplicates are an issue i if we add some CSS styling for padding (for instance) as they will be duplicated and the padding will be summed.

I don't know if the problem come from the vue@cli default setup or from the vuejs3 mounting method.

PS : This duplication is not occuring for vue2 app.

[9mm]

I also have this... what is this originating from?

[ghost]

vue: version 3.0.0-rc.10
@vue/cli: version 4.5.4

The same issue when creating default templates from vue ui.