fix: $effect.pending() should not be affected by uncommitted forks

[waterWang]

Summary

Fixes #18649

$effect.pending() (and boundary pending snippets) should not be affected by async work that happens inside an uncommitted fork(). The fork's work is speculative — it hasn't been committed yet, so it must not surface as pending state. This mirrors how $state.eager() is already unaffected by uncommitted forks.

Root cause

In increment_pending() (packages/svelte/src/internal/client/reactivity/async.js), the boundary's pending count was updated unconditionally:

boundary?.update_pending_count(1, batch);

When a promise is created inside a fork (e.g. an async expression that re-evaluates because state changed within the fork), increment_pending() ran with current_batch.is_fork === true and still bumped the boundary's pending count — even though the fork had not been committed, and the DOM had not (and might never) be updated.

Fix

Capture the is_fork state at the time increment_pending() runs and skip the boundary update when it's an uncommitted fork:

var is_fork = batch.is_fork;

if (!is_fork) {
	boundary?.update_pending_count(1, batch);
}
batch.increment(blocking, effect);

return () => {
	if (!is_fork) {
		boundary?.update_pending_count(-1, batch);
	}
	batch.decrement(blocking, effect);
};

The batch's own pending count is still updated (batch.increment/decrement), so fork().commit() can still await the async work as before. Only the boundary-visible pending state (which powers $effect.pending() and the {#snippet pending()} block) is skipped while the fork is uncommitted.

Tests

• New runtime-runes test async-fork-effect-pending: asserts that $effect.pending() stays 0 (and no pending snippet is shown) when a fork is started but not committed.
• Full runtime-runes + runtime-legacy suites pass: 5966 passed, 0 failed.

[pkg-svelte-dev]

Install the latest version of svelte from 29f4cc7:

pnpm add https://pkg.svelte.dev/svelte/c/29f4cc7348036a7338e49e48c66256e1eb9f2a26

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18651

Note

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 29f4cc7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-18651-svelte.vercel.app/

[dummdidumm]

Thank you, but this solution is incorrect. If the fork commits before the async operation has finished, $effect.pending() will be lower than it should be. Example:

<script>
	import { fork } from 'svelte';
	let count = $state(0);

	function delay(v) {
		return new Promise(r => setTimeout(r, 2000,v))
	}
</script>

{await delay(count)}
{$effect.pending()}
<button onclick={() => {
	const f = fork(() => count++);
	setTimeout(() => f.commit(), 1000);
}}>i</button>

does not show 0 1 after 1 second, just 1 0 after 2.