From 0f4d0b101b2d20247bb0a5167b6f6614e83af561 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 15 Aug 2025 16:46:46 +0200
Subject: [PATCH] fix

---
 .../internal/client/dom/blocks/boundary.js    | 13 ++++++--
 .../src/internal/client/reactivity/batch.js   | 31 +++++++++++++++++--
 .../internal/client/reactivity/deriveds.js    |  5 ++-
 3 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index b7fd9a856c..350a417b8e 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -28,7 +28,7 @@ import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
-import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
+import { Batch, current_batch, effect_pending_updates } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
@@ -69,7 +69,7 @@ export class Boundary {
 	 *
 	 * @type {Batch | null}
 	 */
-	batch = null;
+	#batch = null;
 
 	/** @type {TemplateNode} */
 	#anchor;
@@ -200,6 +200,13 @@ export class Boundary {
 		return !!this.#props.pending;
 	}
 
+	get_batch() {
+		if (current_batch) {
+			this.#batch = current_batch;
+		}
+		return /** @type {Batch} */ (this.#batch);
+	}
+
 	/**
 	 * @param {() => Effect | null} fn
 	 */
@@ -243,7 +250,7 @@ export class Boundary {
 
 		if (this.#pending_count === 0) {
 			this.pending = false;
-			this.batch = null;
+			this.#batch = null;
 
 			if (this.#pending_effect) {
 				pause_effect(this.#pending_effect, () => {
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index b3a3bef731..d102355071 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -184,6 +184,14 @@ export class Batch {
 		/** @type {Map<Source, { v: unknown, wv: number }> | null} */
 		var current_values = null;
 
+		/**
+		 * A batch is superseeded if all of its sources are also in the current batch.
+		 * If the current batch commits, we don't need the old batch anymore.
+		 * This also prevents memory leaks since the old batch will never be committed.
+		 * @type {Batch[]}
+		 */
+		var superseeded_batches = [];
+
 		// if there are multiple batches, we are 'time travelling' —
 		// we need to undo the changes belonging to any batch
 		// other than the current one
@@ -196,15 +204,25 @@ export class Batch {
 				source.v = current;
 			}
 
+			let is_prior_batch = true;
+
 			for (const batch of batches) {
-				if (batch === this) continue;
+				if (batch === this) {
+					is_prior_batch = false;
+					continue;
+				}
+
+				let superseeded = is_prior_batch;
 
 				for (const [source, previous] of batch.#previous) {
 					if (!current_values.has(source)) {
+						superseeded = false;
 						current_values.set(source, { v: source.v, wv: source.wv });
 						source.v = previous;
 					}
 				}
+
+				if (superseeded) superseeded_batches.push(batch);
 			}
 		}
 
@@ -215,6 +233,10 @@ export class Batch {
 		// if we didn't start any new async work, and no async work
 		// is outstanding from a previous flush, commit
 		if (this.#async_effects.length === 0 && this.#pending === 0) {
+			for (const batch of superseeded_batches) {
+				batch.remove();
+			}
+
 			this.#commit();
 
 			var render_effects = this.#render_effects;
@@ -376,6 +398,11 @@ export class Batch {
 		this.#neutered = true;
 	}
 
+	remove() {
+		this.neuter();
+		batches.delete(this);
+	}
+
 	flush() {
 		if (queued_root_effects.length > 0) {
 			flush_effects();
@@ -664,7 +691,7 @@ export function schedule_effect(signal) {
 
 export function suspend() {
 	var boundary = get_pending_boundary();
-	var batch = (boundary.batch ??= /** @type {Batch} */ (current_batch));
+	var batch = boundary.get_batch();
 	var pending = boundary.pending;
 
 	boundary.update_pending_count(1);
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 0ac8582933..8e5d75443d 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -1,5 +1,4 @@
 /** @import { Derived, Effect, Source } from '#client' */
-/** @import { Batch } from './batch.js'; */
 import { DEV } from 'esm-env';
 import {
 	ERROR_VALUE,
@@ -33,7 +32,7 @@ import { 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_deriveds, current_batch } from './batch.js';
+import { batch_deriveds } from './batch.js';
 import { unset_context } from './async.js';
 
 /** @type {Effect | null} */
@@ -131,7 +130,7 @@ export function async_derived(fn, location) {
 
 		prev = promise;
 
-		var batch = (boundary.batch ??= /** @type {Batch} */ (current_batch));
+		var batch = boundary.get_batch();
 		var pending = boundary.pending;
 
 		if (should_suspend) {