fix: store forked derived values (#17212)

We have to take non-tracking contexts into account, especially while in the original `fork(() => ...)` context.

Closes #17206

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/strong-berries-fry.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: store forked derived values

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

@@ -959,12 +959,15 @@ export function fork(fn) {
 
 	var batch = Batch.ensure();
 	batch.is_fork = true;
+	batch_values = new Map();
 
 	var committed = false;
 	var settled = batch.settled();
 
 	flushSync(fn);
 
+	batch_values = null;
+
 	// revert state changes
 	for (var [source, value] of batch.previous) {
 		source.v = value;

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

@@ -378,7 +378,7 @@ export function update_derived(derived) {
 	if (batch_values !== null) {
 		// only cache the value if we're in a tracking context, otherwise we won't
 		// clear the cache in `mark_reactions` when dependencies are updated
-		if (effect_tracking()) {
+		if (effect_tracking() || current_batch?.is_fork) {
 			batch_values.set(derived, value);
 		}
 	} else {

packages/svelte/src/internal/client/runtime.js

@@ -44,7 +44,13 @@ import {
 	set_dev_stack
 } from './context.js';
 import * as w from './warnings.js';
-import { Batch, batch_values, flushSync, schedule_effect } from './reactivity/batch.js';
+import {
+	Batch,
+	batch_values,
+	current_batch,
+	flushSync,
+	schedule_effect
+} from './reactivity/batch.js';
 import { handle_error } from './error-handling.js';
 import { UNINITIALIZED } from '../../constants.js';
 import { captured_signals } from './legacy.js';
@@ -612,7 +618,10 @@ export function get(signal) {
 
 			return value;
 		}
-	} else if (is_derived && !batch_values?.has(signal)) {
+	} else if (
+		is_derived &&
+		(!batch_values?.has(signal) || (current_batch?.is_fork && !effect_tracking()))
+	) {
 		derived = /** @type {Derived} */ (signal);
 
 		if (is_dirty(derived)) {

packages/svelte/tests/runtime-runes/samples/fork-derived-value-immediate/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target, logs }) {
+		const fork = target.querySelector('button');
+
+		fork?.click();
+		flushSync();
+		assert.deepEqual(logs, [1, 2]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-value-immediate/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	import { fork } from "svelte";
+
+	let state = $state(0);
+	let count = $derived(state);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		state++;
+		console.log(count);
+		state++;
+		console.log(count);
+	});
+}}>fork</button>