fix: ensure fork always accesses correct values

Not all batches will flush right after being activated, some will be activated and then `get` is called on a signal. In that case the value was wrong because we did not apply the changes of that batch. By doing `this.apply()` during `activate()` we ensure we do, which fixes (among other things, likely) a forking bug where old values where sneaking in.

Fixes #17079

.changeset/eight-news-laugh.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure fork always accesses correct values

packages/svelte/src/internal/client/reactivity/batch.js

@@ -15,7 +15,8 @@ import {
 	DERIVED,
 	BOUNDARY_EFFECT,
 	EAGER_EFFECT,
-	HEAD_EFFECT
+	HEAD_EFFECT,
+	ERROR_VALUE
 } from '#client/constants';
 import { async_mode_flag } from '../../flags/index.js';
 import { deferred, define_property } from '../../shared/utils.js';
@@ -285,12 +286,16 @@ export class Batch {
 			this.previous.set(source, value);
 		}
 
-		this.current.set(source, source.v);
-		batch_values?.set(source, source.v);
+		// Don't save this as it would mean it's not thrown in the `runtime.js#get` function
+		if ((source.f & ERROR_VALUE) === 0) {
+			this.current.set(source, source.v);
+			batch_values?.set(source, source.v);
+		}
 	}
 
 	activate() {
 		current_batch = this;
+		this.apply();
 	}
 
 	deactivate() {
@@ -492,7 +497,7 @@ export class Batch {
 	}
 
 	apply() {
-		if (!async_mode_flag || batches.size === 1) return;
+		if (!async_mode_flag || (!this.is_fork && batches.size === 1)) return;
 
 		// if there are multiple batches, we are 'time travelling' —
 		// we need to override values with the ones in this batch...

packages/svelte/tests/runtime-runes/samples/async-fork-if/Child.svelte

@@ -0,0 +1,8 @@
+<script>
+	let { x } = $props();
+	console.log(x);
+	await Promise.resolve();
+	console.log(x);
+</script>
+
+{x}

packages/svelte/tests/runtime-runes/samples/async-fork-if/_config.js

@@ -0,0 +1,12 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const btn = target.querySelector('button');
+
+		btn?.click();
+		await new Promise((r) => setTimeout(r, 2));
+		assert.htmlEqual(target.innerHTML, `<button>fork</button> universe`);
+		assert.deepEqual(logs, ['universe', 'universe']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-if/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+	let x = $state('world');
+</script>
+
+<button onclick={async () => {
+	const f = fork(() => {
+		x = 'universe'
+	});
+	await new Promise(r => setTimeout(r));
+	f.commit();
+}}>fork</button>
+
+{#if x === 'universe'}
+	<Child {x} />
+{/if}