Steps

1. yarn
2. yarn dev
3. Check browser console:

[Vue warn]: Hydration node mismatch:
- Client vnode: div
- Server rendered DOM: <componenta>​</componenta>​
  at <ComponentA>
  at <Home onVnodeUnmounted=fn<onVnodeUnmounted> ref=Ref< undefined > >
  at <RouterView>
  at <App>

Also check terminal output:

[Vue warn]: resolveComponent can only be used in render() or setup().

4. Remove <my-x /> from template OR replace it with <div />. Notice no hydration error.

Point of interest

/src/pages/Home.vue. The rest of the files don't matter.