From 302dff234b000df1ba9c6006624b66fc473008a1 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 1 Jun 2025 21:05:09 -0400
Subject: [PATCH] don't write values to deriveds when time travelling

---
 .../src/internal/client/reactivity/batch.js   | 54 +++++++++++------
 .../svelte/src/internal/client/runtime.js     | 21 ++++++-
 .../async-with-sync-derived/_config.js        | 59 +++++++++++++++++++
 .../async-with-sync-derived/main.svelte       | 19 ++++++
 4 files changed, 133 insertions(+), 20 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index d705846477..9e1fcf7075 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -1,4 +1,4 @@
-/** @import { Effect, Source } from '#client' */
+/** @import { Derived, Effect, Source } from '#client' */
 import { CLEAN, DIRTY } from '#client/constants';
 import {
 	flush_queued_effects,
@@ -23,7 +23,8 @@ function update_pending() {
 	internal_set(pending, batches.size > 0);
 }
 
-let uid = 1;
+/** @type {Map<Derived, any> | null} */
+export let batch_deriveds = null;
 
 export class Batch {
 	/** @type {Map<Source, any>} */
@@ -60,21 +61,34 @@ export class Batch {
 	process(root_effects) {
 		set_queued_root_effects([]);
 
-		/** @type {Map<Source, { v: unknown, wv: number }>} */
-		var current_values = new Map();
+		/** @type {Map<Source, { v: unknown, wv: number }> | null} */
+		var current_values = null;
+		var time_travelling = false;
 
-		for (const [source, current] of this.#current) {
-			current_values.set(source, { v: source.v, wv: source.wv });
-			source.v = current;
+		for (const batch of batches) {
+			if (batch !== this) {
+				time_travelling = true;
+				break;
+			}
 		}
 
-		for (const batch of batches) {
-			if (batch === this) continue;
+		if (time_travelling) {
+			current_values = new Map();
+			batch_deriveds = new Map();
+
+			for (const [source, current] of this.#current) {
+				current_values.set(source, { v: source.v, wv: source.wv });
+				source.v = current;
+			}
 
-			for (const [source, previous] of batch.#previous) {
-				if (!current_values.has(source)) {
-					current_values.set(source, { v: source.v, wv: source.wv });
-					source.v = previous;
+			for (const batch of batches) {
+				if (batch === this) continue;
+
+				for (const [source, previous] of batch.#previous) {
+					if (!current_values.has(source)) {
+						current_values.set(source, { v: source.v, wv: source.wv });
+						source.v = previous;
+					}
 				}
 			}
 		}
@@ -144,12 +158,16 @@ export class Batch {
 			for (const e of this.effects) set_signal_status(e, CLEAN);
 		}
 
-		for (const [source, { v, wv }] of current_values) {
-			// reset the source to the current value (unless
-			// it got a newer value as a result of effects running)
-			if (source.wv <= wv) {
-				source.v = v;
+		if (current_values) {
+			for (const [source, { v, wv }] of current_values) {
+				// reset the source to the current value (unless
+				// it got a newer value as a result of effects running)
+				if (source.wv <= wv) {
+					source.v = v;
+				}
 			}
+
+			batch_deriveds = null;
 		}
 
 		for (const effect of this.async_effects) {
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 39b27f9f73..76c18e6362 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -39,6 +39,7 @@ import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
 import {
 	destroy_derived_effects,
+	execute_derived,
 	from_async_derived,
 	recent_async_deriveds,
 	update_derived
@@ -57,7 +58,7 @@ import {
 import { Boundary } from './dom/blocks/boundary.js';
 import * as w from './warnings.js';
 import { is_firefox } from './dom/operations.js';
-import { current_batch, Batch } from './reactivity/batch.js';
+import { current_batch, Batch, batch_deriveds } from './reactivity/batch.js';
 import { log_effect_tree, root } from './dev/debug.js';
 
 // Used for DEV time error handling
@@ -986,7 +987,10 @@ export function get(signal) {
 		}
 	}
 
-	if (is_derived) {
+	// if this is a derived, we may need to update it, but
+	// not if `batch_deriveds` is not null (meaning we're
+	// currently time travelling))
+	if (is_derived && batch_deriveds === null) {
 		derived = /** @type {Derived} */ (signal);
 
 		if (check_dirtiness(derived)) {
@@ -1032,6 +1036,19 @@ export function get(signal) {
 		return old_values.get(signal);
 	}
 
+	// if we're time travelling, we don't want to update the
+	// intrinsic value of the derived — we want to compute it
+	// once and stash it for the duration of batch processing
+	if (is_derived && batch_deriveds !== null) {
+		derived = /** @type {Derived} */ (signal);
+
+		if (!batch_deriveds.has(derived)) {
+			batch_deriveds.set(derived, execute_derived(derived));
+		}
+
+		return batch_deriveds.get(derived);
+	}
+
 	return signal.v;
 }
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/_config.js b/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/_config.js
new file mode 100644
index 0000000000..c09d448f9c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/_config.js
@@ -0,0 +1,59 @@
+import { flushSync, settled, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<p>loading...</p>`,
+
+	async test({ assert, target }) {
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>log</button>
+				<button>x 1</button>
+				<button>other 1</button>
+				<p>1</p>
+				<p>1</p>
+				<p>1</p>
+			`
+		);
+
+		const [log, x, other] = target.querySelectorAll('button');
+
+		flushSync(() => x.click());
+		flushSync(() => other.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>log</button>
+				<button>x 1</button>
+				<button>other 2</button>
+				<p>1</p>
+				<p>1</p>
+				<p>1</p>
+			`
+		);
+
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>log</button>
+				<button>x 2</button>
+				<button>other 2</button>
+				<p>2</p>
+				<p>2</p>
+				<p>2</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/main.svelte
new file mode 100644
index 0000000000..764007e082
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-with-sync-derived/main.svelte
@@ -0,0 +1,19 @@
+<script>
+	let x = $state(1);
+	let y = $derived(x);
+	let other = $state(1);
+</script>
+
+<svelte:boundary>
+	<button onclick={() => console.log({ x, y })}>log</button>
+	<button onclick={() => x += 1}>x {x}</button>
+	<button onclick={() => other += 1}>other {other}</button>
+
+	<p>{x}</p>
+	<p>{await x}</p>
+	<p>{y}</p>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>