+page mounts twice on one navigation when a persistent layout renders an outgoing route's teleported snippet

[torrfura]

Describe the bug

With compilerOptions.experimental.async: true, a destination route's +page is instantiated twice on a single client-side navigation — its onMount runs twice, and the first instance is created, rendered, then torn down again while still reactively alive (which surfaces as derived_inert warnings in a real app). With the flag off, it mounts once.

There are no await expressions in the reproduction, so this is not about async data or update coordination — only the experimental.async rendering machinery's mount/teardown.

Three conditions are required together (removing any one → single mount):

1. A persistent layout renders a snippet teleported up from a descendant — the descendant registers a snippet via a context store, and the persistent layout renders it elsewhere in its own tree (a portal); the layout survives the navigation.
2. The outgoing route teleports its own child route ({@render children()}) through that persistent layout.
3. The destination is a 2-level route (+layout + +page).

Only the destination +page leaf doubles; its +layout and the persistent host mount once. It is specifically the persistent host rendering the outgoing route's teleported snippet, concurrent with the navigation — deleting just that {@render slot.teleported()} (keeping everything else) makes it mount once, and rendering a host-owned snippet in the same spot does not trigger it.

Reproduction

https://github.com/torrfura/sveltekit-async-double-mount

npm install && npm run dev   # http://localhost:5173

Open the console, click /teleporter, then click /nested. The single /teleporter → /nested navigation mounts nested/+page twice. The on-page panel resets per navigation and shows a verdict, so there's nothing cumulative to misread.

Controls (each removes one condition → single mount):

Navigation	Outgoing teleports its child?	Destination	dest +page mounts
/teleporter → /nested	yes	2-level	twice
/teleporter → /flat	yes	1-level	once
/inline → /nested	no	2-level	once

Logs

### Logs


🐛 DOUBLE MOUNT: nested/+page mounted 2× in one navigation → /nested

System Info

System:
  OS: macOS 26.5.1
Binaries:
  Node: 24.14.0
  npm: 11.9.0
npmPackages:
  svelte: 5.56.3
  @sveltejs/kit: 3.0.0-next.4
  @sveltejs/vite-plugin-svelte: 7.1.2
  vite: 8.0.16

Severity

annoyance

Additional Information

This stems from ideas like these: #627 But in our case it's about layout orchestration and animation using view transitions.

[eftenow]

you can wrap the slot cleanup in untrack inside teleport.svelte. The persistent layout reacts to slot.teleported going undefined mid-navigation, which is what races with the async mount. Wrapping it in untrack lets the state clear without rerendering, so the scheduler doesn't double-instantiate nested/+page

[torrfura]

Thanks for the suggestion! Gave the untrack a shot but it didn't seem to budge it — nested/+page still mounts twice.

My (very possibly wrong) hunch is that since the cleanup is a write rather than a read, untrack doesn't really stop the persistent layout's {#if slot.teleported} from reacting — so it still seems to re-render mid-navigation? Could easily be misreading it.

The thing that did seem to help thanks to you is clearing the slot at nav-start (an onNavigate up in the persistent host) instead of in the teleport's unmount cleanup — so the outgoing teleported bit is gone before the destination gets instantiated, and surviving teleports just re-assert themselves. No idea if that's the "correct" mental model or if we're just accidentally dodging it, but it's held up quite reliably in this repro and our real heavily View-Transition driven app.

Added this (plus a little persist scenario) on a branch in case it's any use: https://github.com/torrfura/sveltekit-async-double-mount/tree/fix-navstart-clear-reassert

Happy to poke at it more / try another repro angle if that'd help. Thanks again!