fix: store forked derived values

.changeset/strong-berries-fry.md

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

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

@@ -68,6 +68,14 @@ export let previous_batch = null;
  */
 export let batch_values = null;
 
+/**
+ * When time travelling (i.e. working in one batch, while other batches
+ * still have ongoing work), we ignore the real values of affected
+ * signals in favour of their values within the batch
+ * @type {Map<Value, any> | null}
+ */
+export let forked_derived_values = null;
+
 // TODO this should really be a property of `batch`
 /** @type {Effect[]} */
 let queued_root_effects = [];
@@ -962,8 +970,12 @@ export function fork(fn) {
 	var committed = false;
 	var settled = batch.settled();
 
+	forked_derived_values = new Map();
+
 	flushSync(fn);
 
+	forked_derived_values = null;
+
 	// revert state changes
 	for (var [source, value] of batch.previous) {
 		source.v = value;

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

@@ -34,7 +34,7 @@ import { async_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { Boundary } from '../dom/blocks/boundary.js';
 import { component_context } from '../context.js';
 import { UNINITIALIZED } from '../../../constants.js';
-import { batch_values, current_batch } from './batch.js';
+import { batch_values, current_batch, forked_derived_values } from './batch.js';
 import { unset_context } from './async.js';
 import { deferred } from '../../shared/utils.js';
 
@@ -360,8 +360,10 @@ export function update_derived(derived) {
 		// the underlying value will be updated when the fork is committed.
 		// otherwise, the next time we get here after a 'real world' state
 		// change, `derived.equals` may incorrectly return `true`
-		if (!current_batch?.is_fork) {
+		if (!forked_derived_values) {
 			derived.v = value;
+		} else {
+			forked_derived_values.set(derived, value);
 		}
 
 		derived.wv = increment_write_version();

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

@@ -43,7 +43,13 @@ 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,
+	flushSync,
+	forked_derived_values,
+	schedule_effect
+} from './reactivity/batch.js';
 import { handle_error } from './error-handling.js';
 import { UNINITIALIZED } from '../../constants.js';
 import { captured_signals } from './legacy.js';
@@ -621,6 +627,10 @@ export function get(signal) {
 		if (is_updating_effect && effect_tracking() && (derived.f & CONNECTED) === 0) {
 			reconnect(derived);
 		}
+
+		if (forked_derived_values?.has(derived)) {
+			return forked_derived_values.get(derived);
+		}
 	}
 
 	if (batch_values?.has(signal)) {

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

@@ -0,0 +1,15 @@
+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');
+
+		flushSync(() => {
+			fork?.click();
+		});
+
+		assert.deepEqual(logs, [1]);
+	}
+});

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

@@ -0,0 +1,13 @@
+<script>
+	import { fork } from "svelte";
+
+	let state = $state(0);
+	let count = $derived(state);
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		state++;
+		console.log(count);
+	});
+}}>fork</button>