OnMount lifecycle hook run in wrong sequence between parent and child components

[Tbutnyakov]

The problem
Since 3.6.4 version we have a wrong sequence for onMount lifecycle hook.
It run onMount callback first on a child component, instead of parent.

To Reproduce
https://svelte.dev/repl/9eb25e5039844a9588cd96bc7380e672?version=3.6.4

Expected behaviour
Take a look at the console.
Im expect this order for hooks calls:

onMount Parent
onMount Child

Instead we got:

onMount Child
onMount Parent

[tanhauhau]

yes the child get mounted first, before the parent.

this is intended.

see #2281

[direct-fuel-injection]

yes the child get mounted first, before the parent.

this is intended.

see #2281

this is not the same behavior

[Tbutnyakov]

@tanhauhau i saw that issue.
It seems like when you fixed that problem you created a new. But much more bigger.

How can you mount an element to a void, can you explain pls?
Where do you mount a child component if you didn't mount its parent yet?