experimental.async: nested dynamic component mounts twice when swapped while a persistent ancestor renders the outgoing subtree's teleported snippet

Describe the bug

With compilerOptions.experimental.async: true, replacing two nested dynamic components in one synchronous $state assignment instantiates the new leaf component twice. Its onMount runs twice, and one of the instances is transient, torn down right after mounting while still reactively alive (in a larger app this also surfaces as derived_inert warnings). There are no await expressions anywhere in the repro.

Three conditions are required together, removing any one of them gives a single mount:

1. A persistent ancestor renders a snippet that a descendant in the outgoing subtree registered via context (a portal), and renders it elsewhere in its own tree.
2. The outgoing subtree teleports its children through that ancestor.
3. The swap replaces dynamic <Component>s (two levels, the new leaf is the one that doubles). A static {#if} swap of the same components mounts once.

With experimental.async: false everything mounts once.

Extracted from sveltejs/kit#16166. It reproduces without SvelteKit (repro below is plain Vite + Svelte), so it is not a kit bug. Kit is the natural trigger because its client swaps the route component tree exactly like this on every navigation, and the snippet-teleport pattern is the standard workaround for sveltejs/kit#627.

Reproduction

https://github.com/Nic-Polumeyv/svelte-async-double-mount

npm install && npm run dev

Click navigate. The on-page verdict and console show nested/page mounting twice. The README has the full ablation table.

Logs

[mount] nested/page
[mount] nested/layout
DOUBLE MOUNT: nested/page mounted 2x

System Info

svelte: 5.56.6
@sveltejs/vite-plugin-svelte: 7.1.2
vite: 8.0.16
node: 22.23.1 (linux)

Also reproduces through SvelteKit on macOS per the original kit report (kit 3.0.0-next.4 and next.8).

Severity

annoyance