fix: don't commit a navigation superseded while onNavigate is pending

[khughitt]

navigate() checks navigation_token once its route has loaded, then awaits onNavigate callbacks and commits without checking again. When a callback returns a promise (the view transition recipe from the docs holds the navigation until the transition's update callback runs), a newer navigation can start during that wait, and the superseded one still sets current and applies its render tree when its callbacks settle. This adds the same token check after the onNavigate callbacks as the one after load.

With experimental.forkPreloads, navigate() has already taken the preload fork out of load_cache at that point, so nothing else can reach it. The new abort path therefore discards the fork itself, the same way discard_load_cache() does.

Functions returned by onNavigate must also not outlive an aborted navigation, or they run when the next navigation completes. run_on_navigate_callbacks() used to register them before the check. It now returns them, and the caller registers them only if the navigation is still current. finish_navigation() also receives them and removes them when it aborts, which covers a navigation superseded while its render settles. They stay registered before the commit, so they still run in the same order relative to other afterNavigate callbacks. Each registration wraps the returned functions in entries of its own. after_navigate_callbacks is a Set, so if two navigations' onNavigate calls returned the same function object, removing the aborted navigation's registration would otherwise remove the newer navigation's too.

A shallow goto() has the same gap in update_state(): superseded while onNavigate was pending, it still applied its page state and registered its functions. It now gets the same check. As with a full navigation aborted at that point, its history entry has already been pushed and stays; the check only stops the page update and the registration.

On version-3 the newer navigation still commits afterwards, so the result is transient: the superseded page mounts, renders and runs its effects, and its onNavigate return value is registered as an afterNavigate callback, just before the newest navigation replaces it.

On 2.x (2.70.3, main) the same gap leaves the wrong page in place. There, a navigation only sends the data_N props whose node data differs from current at load time. If a superseded navigation commits between a newer navigation's load and its commit, the newer navigation's props are diffed against a stale current: it commits without data_N for a page whose data didn't change relative to that current. The URL then shows the newest route while the page keeps the superseded route's data. We hit this with rapid link clicks and Back under view transitions. I'm happy to open a backport against main if that's wanted.

No existing issue covers this. #12809 involved a pending onNavigate too, but that was a different symptom.

Test

navigation-lifecycle/on-navigate-superseded/[id] holds every navigation in onNavigate. The test starts a navigation to b, then a newer one back to a, releases the superseded one first, and asserts that b never renders and that only the committed navigation's onNavigate return value runs. It fails on version-3 without the change (Received: "b"), fails without the registration change (the aborted navigation's function runs), and passes 20/20 in dev and build with both. A second case runs the same sequence with a shallow goto() as the superseded navigation. Without the change, the superseded navigation's state is applied (Received: "active").

In async, which enables forkPreloads, fork/superseded preloads a page that subscribes to a store counting its subscribers. The navigation to that page is held in onNavigate and superseded, and the test asserts that releasing it drops the count back to 0, meaning the fork was discarded. Without the discard it fails (Received: 1), and it passes 20/20 in dev and build with it.

Also in async, navigation-settle holds a page's render with a top-level await, and a newer navigation starts before it settles. The test asserts that only the newer navigation's onNavigate return value runs. Without removing them on that abort it fails (the held navigation's function runs), and it passes 20/20 in dev and build with it. The page also returns one shared function object from every navigation, and the test asserts that the newer navigation still runs it. Without the per-registration entries, the aborted navigation's cleanup removes it.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from da3cc3d:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/da3cc3dd97cccaa76aabec54cf66c65d964059e0

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17200

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

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Latest commit: da3cc3d

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[jamalkamaladdin]

run_on_navigate_callbacks registers functions returned from onNavigate before this check, and this return leaves them in after_navigate_callbacks. A function returned for an aborted navigation runs when the next navigation commits, although its page never rendered.

[khughitt]

Confirmed, the aborted navigation's returned function ran when the next one completed. run_on_navigate_callbacks() now returns those functions instead of registering them, and navigate() registers them after the token check, so an aborted navigation never registers its function. The shallow-routing caller registers them immediately, as before. The basics test now also asserts that only the committed navigation's function runs. It failed without this change, and passes with it.