From b8893f95d9ad3f805ee637303bfc9b407baea651 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Jul 2025 17:50:35 +0200
Subject: [PATCH 01/19] - add state changes resulting from an $effect to a
 separate new batch - schedule rerunning effects based on the sources that are
 dirty, not just rerunning them all blindly (excempting async effects which
 will have run by that time already)

---
 .../src/internal/client/reactivity/batch.js   | 176 ++++++++++--------
 .../src/internal/client/reactivity/sources.js |   7 +-
 2 files changed, 103 insertions(+), 80 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index ec082bb595ff..6b7258ec4817 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -28,7 +28,7 @@ import * as e from '../errors.js';
 import { flush_tasks } from '../dom/task.js';
 import { DEV } from 'esm-env';
 import { invoke_error_boundary } from '../error-handling.js';
-import { old_values } from './sources.js';
+import { mark_reactions, old_values } from './sources.js';
 import { unlink_effect } from './effects.js';
 import { unset_context } from './async.js';
 
@@ -70,13 +70,15 @@ let last_scheduled_effect = null;
 
 let is_flushing = false;
 
+let flushing_sync = false;
+
 export class Batch {
 	/**
 	 * The current values of any sources that are updated in this batch
 	 * They keys of this map are identical to `this.#previous`
 	 * @type {Map<Source, any>}
 	 */
-	#current = new Map();
+	current = new Map();
 
 	/**
 	 * The values of any sources that are updated in this batch _before_ those updates took place.
@@ -156,7 +158,7 @@ export class Batch {
 	 *
 	 * @param {Effect[]} root_effects
 	 */
-	#process(root_effects) {
+	process(root_effects) {
 		queued_root_effects = [];
 
 		/** @type {Map<Source, { v: unknown, wv: number }> | null} */
@@ -169,7 +171,7 @@ export class Batch {
 			current_values = new Map();
 			batch_deriveds = new Map();
 
-			for (const [source, current] of this.#current) {
+			for (const [source, current] of this.current) {
 				current_values.set(source, { v: source.v, wv: source.wv });
 				source.v = current;
 			}
@@ -300,7 +302,7 @@ export class Batch {
 			this.#previous.set(source, value);
 		}
 
-		this.#current.set(source, source.v);
+		this.current.set(source, source.v);
 	}
 
 	activate() {
@@ -346,49 +348,7 @@ export class Batch {
 	}
 
 	flush_effects() {
-		var was_updating_effect = is_updating_effect;
-		is_flushing = true;
-
-		try {
-			var flush_count = 0;
-			set_is_updating_effect(true);
-
-			while (queued_root_effects.length > 0) {
-				if (flush_count++ > 1000) {
-					if (DEV) {
-						var updates = new Map();
-
-						for (const source of this.#current.keys()) {
-							for (const [stack, update] of source.updated ?? []) {
-								var entry = updates.get(stack);
-
-								if (!entry) {
-									entry = { error: update.error, count: 0 };
-									updates.set(stack, entry);
-								}
-
-								entry.count += update.count;
-							}
-						}
-
-						for (const update of updates.values()) {
-							// eslint-disable-next-line no-console
-							console.error(update.error);
-						}
-					}
-
-					infinite_loop_guard();
-				}
-
-				this.#process(queued_root_effects);
-				old_values.clear();
-			}
-		} finally {
-			is_flushing = false;
-			set_is_updating_effect(was_updating_effect);
-
-			last_scheduled_effect = null;
-		}
+		flush_effects();
 	}
 
 	/**
@@ -412,19 +372,8 @@ export class Batch {
 		this.#pending -= 1;
 
 		if (this.#pending === 0) {
-			for (const e of this.#render_effects) {
-				set_signal_status(e, DIRTY);
-				schedule_effect(e);
-			}
-
-			for (const e of this.#effects) {
-				set_signal_status(e, DIRTY);
-				schedule_effect(e);
-			}
-
-			for (const e of this.#block_effects) {
-				set_signal_status(e, DIRTY);
-				schedule_effect(e);
+			for (const source of this.current.keys()) {
+				mark_reactions(source, DIRTY, false);
 			}
 
 			this.#render_effects = [];
@@ -487,32 +436,88 @@ export function flushSync(fn) {
 		e.flush_sync_in_effect();
 	}
 
-	var result;
+	var prev_flushing_sync = flushing_sync;
+	flushing_sync = true;
 
-	const batch = Batch.ensure(false);
+	try {
+		var result;
 
-	if (fn) {
-		batch.flush_effects();
+		const batch = Batch.ensure(false);
 
-		result = fn();
-	}
+		if (fn) {
+			batch.flush_effects();
 
-	while (true) {
-		flush_tasks();
+			result = fn();
+		}
+
+		while (true) {
+			flush_tasks();
+
+			if (queued_root_effects.length === 0) {
+				// TODO this might need adjustment
+				if (batch === current_batch) {
+					batch.flush();
+				}
+
+				// this would be reset in `batch.flush_effects()` but since we are early returning here,
+				// we need to reset it here as well in case the first time there's 0 queued root effects
+				last_scheduled_effect = null;
 
-		if (queued_root_effects.length === 0) {
-			if (batch === current_batch) {
-				batch.flush();
+				return /** @type {T} */ (result);
 			}
 
-			// this would be reset in `batch.flush_effects()` but since we are early returning here,
-			// we need to reset it here as well in case the first time there's 0 queued root effects
-			last_scheduled_effect = null;
+			batch.flush_effects();
+		}
+	} finally {
+		flushing_sync = prev_flushing_sync;
+	}
+}
 
-			return /** @type {T} */ (result);
+function flush_effects() {
+	var was_updating_effect = is_updating_effect;
+	is_flushing = true;
+
+	try {
+		var flush_count = 0;
+		var batch = /** @type {Batch} */ (current_batch);
+		set_is_updating_effect(true);
+
+		while (queued_root_effects.length > 0) {
+			if (flush_count++ > 1000) {
+				if (DEV) {
+					var updates = new Map();
+
+					for (const source of batch.current.keys()) {
+						for (const [stack, update] of source.updated ?? []) {
+							var entry = updates.get(stack);
+
+							if (!entry) {
+								entry = { error: update.error, count: 0 };
+								updates.set(stack, entry);
+							}
+
+							entry.count += update.count;
+						}
+					}
+
+					for (const update of updates.values()) {
+						// eslint-disable-next-line no-console
+						console.error(update.error);
+					}
+				}
+
+				infinite_loop_guard();
+			}
+
+			batch = /** @type {Batch} */ (current_batch);
+			batch.process(queued_root_effects);
+			old_values.clear();
 		}
+	} finally {
+		is_flushing = false;
+		set_is_updating_effect(was_updating_effect);
 
-		batch.flush_effects();
+		last_scheduled_effect = null;
 	}
 }
 
@@ -545,6 +550,7 @@ function flush_queued_effects(effects) {
 		if ((effect.f & (DESTROYED | INERT)) === 0) {
 			if (is_dirty(effect)) {
 				var wv = write_version;
+				var current_size = /** @type {Batch} */ (current_batch).current.size;
 
 				update_effect(effect);
 
@@ -568,6 +574,22 @@ function flush_queued_effects(effects) {
 				// if state is written in a user effect, abort and re-schedule, lest we run
 				// effects that should be removed as a result of the state change
 				if (write_version > wv && (effect.f & USER_EFFECT) !== 0) {
+					// Work needs to happen in a separate batch, else prior sources would be mixed with
+					// newly updated sources, which could lead to infinite loops when effects run over and over again.
+					// We need to bring over the just written sources though to correctly mark the right reactions as dirty.
+					var old_batch = /** @type {Batch} */ (current_batch);
+					batches.delete(old_batch);
+					current_batch = null;
+					var new_batch = Batch.ensure(!flushing_sync);
+					var current_idx = 0;
+					// We're taking advantage of the spec here which says that entries in a Map are traversed by insertion order
+					for (const source of old_batch.current) {
+						if (current_idx >= current_size) {
+							new_batch.capture(source[0], source[1]);
+						}
+						current_idx++;
+					}
+					i++;
 					break;
 				}
 			}
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index f6b14f3360de..9600714feb71 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -301,9 +301,10 @@ export function increment(source) {
 /**
  * @param {Value} signal
  * @param {number} status should be DIRTY or MAYBE_DIRTY
+ * @param {boolean} schedule_async
  * @returns {void}
  */
-function mark_reactions(signal, status) {
+export function mark_reactions(signal, status, schedule_async = true) {
 	var reactions = signal.reactions;
 	if (reactions === null) return;
 
@@ -324,13 +325,13 @@ function mark_reactions(signal, status) {
 		}
 
 		// don't set a DIRTY reaction to MAYBE_DIRTY
-		if ((flags & DIRTY) === 0) {
+		if ((flags & DIRTY) === 0 && (schedule_async || (flags & ASYNC) === 0)) {
 			set_signal_status(reaction, status);
 		}
 
 		if ((flags & DERIVED) !== 0) {
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
-		} else if ((flags & DIRTY) === 0) {
+		} else if ((flags & DIRTY) === 0 && (schedule_async || (flags & ASYNC) === 0)) {
 			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}

From 26f46bac35eb5853eacb33096abd7c2254f102d7 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Jul 2025 17:52:53 +0200
Subject: [PATCH 02/19] test

---
 .../async-effect-triggers-await/_config.js    | 32 +++++++++++++++++++
 .../async-effect-triggers-await/main.svelte   | 24 ++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/_config.js
new file mode 100644
index 000000000000..c551cc6b8c39
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/_config.js
@@ -0,0 +1,32 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [increment] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>1</p>
+				<p>1</p>
+			`
+		);
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>2</p>
+				<p>2</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/main.svelte
new file mode 100644
index 000000000000..153fe03f0d89
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-triggers-await/main.svelte
@@ -0,0 +1,24 @@
+<script lang="ts">
+	let data = $state(Promise.resolve(0));
+
+	let count = $state(0);
+	let unrelated = $state(0);
+
+	$effect(() => {
+		data = Promise.resolve(count)
+		unrelated = count;
+	});
+</script>
+
+<svelte:boundary>
+	<button onclick={() => count += 1}>increment</button>
+	<p>{JSON.stringify((await data), null, 2)}</p>
+	{#if true}
+		<!-- inside if block to force it into a different render effect -->
+		<p>{unrelated}</p>
+	{/if}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>

From 0c0662bbdca1159c3630f43c5d4d7ef194e4cf89 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Jul 2025 20:21:03 +0200
Subject: [PATCH 03/19] better fix

---
 .../src/internal/client/reactivity/batch.js   | 31 +++++++++----------
 .../src/internal/client/reactivity/sources.js |  4 +--
 2 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 6e748ffbdf9e..2c959855002c 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -70,7 +70,7 @@ let last_scheduled_effect = null;
 
 let is_flushing = false;
 
-let flushing_sync = false;
+export let flushing_sync = false;
 
 export class Batch {
 	/**
@@ -204,9 +204,22 @@ export class Batch {
 			this.#effects = [];
 			this.#block_effects = [];
 
+			// If sources are written to, then work needs to happen in a separate batch, else prior sources would be mixed with
+			// newly updated sources, which could lead to infinite loops when effects run over and over again.
+			current_batch = null;
+
 			flush_queued_effects(render_effects);
 			flush_queued_effects(effects);
 
+			// Reinstate the current batch if there was no new one created, as `process()` runs in a loop in `flush_effects()`.
+			// That method expects `current_batch` to be set, and could run the loop again if effects result in new effects
+			// being scheduled but without writes happening in which case no new batch is created.
+			if (current_batch === null) {
+				current_batch = this;
+			} else {
+				batches.delete(this);
+			}
+
 			this.#deferred?.resolve();
 		} else {
 			// otherwise mark effects clean so they get scheduled on the next run
@@ -551,7 +564,6 @@ function flush_queued_effects(effects) {
 
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
 			var wv = write_version;
-			var current_size = /** @type {Batch} */ (current_batch).current.size;
 
 			update_effect(effect);
 
@@ -575,21 +587,6 @@ function flush_queued_effects(effects) {
 			// if state is written in a user effect, abort and re-schedule, lest we run
 			// effects that should be removed as a result of the state change
 			if (write_version > wv && (effect.f & USER_EFFECT) !== 0) {
-				// Work needs to happen in a separate batch, else prior sources would be mixed with
-				// newly updated sources, which could lead to infinite loops when effects run over and over again.
-				// We need to bring over the just written sources though to correctly mark the right reactions as dirty.
-				var old_batch = /** @type {Batch} */ (current_batch);
-				batches.delete(old_batch);
-				current_batch = null;
-				var new_batch = Batch.ensure(!flushing_sync);
-				var current_idx = 0;
-				// We're taking advantage of the spec here which says that entries in a Map are traversed by insertion order
-				for (const source of old_batch.current) {
-					if (current_idx >= current_size) {
-						new_batch.capture(source[0], source[1]);
-					}
-					current_idx++;
-				}
 				break;
 			}
 		}
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 9600714feb71..41fda858a52b 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -33,7 +33,7 @@ import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { get_stack, tag_proxy } from '../dev/tracing.js';
 import { component_context, is_runes } from '../context.js';
-import { Batch, schedule_effect } from './batch.js';
+import { Batch, flushing_sync, schedule_effect } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 
@@ -179,7 +179,7 @@ export function internal_set(source, value) {
 
 		source.v = value;
 
-		const batch = Batch.ensure();
+		const batch = Batch.ensure(!flushing_sync);
 		batch.capture(source, old_value);
 
 		if (DEV) {

From 4cb484709f39edfb1843b3a5834fa6278a990d88 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Jul 2025 21:13:41 +0200
Subject: [PATCH 04/19] tests

---
 .../_config.js                                | 26 ++++++++++++++++++
 .../main.svelte                               | 27 +++++++++++++++++++
 .../binding-update-while-focused-2/_config.js | 24 +++++++++++++++++
 .../main.svelte                               | 21 +++++++++++++++
 4 files changed, 98 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/_config.js b/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/_config.js
new file mode 100644
index 000000000000..782ae945f9c3
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/_config.js
@@ -0,0 +1,26 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [input] = target.querySelectorAll('input');
+
+		input.focus();
+		input.value = '3';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		await tick();
+
+		assert.equal(input.value, '3');
+		assert.htmlEqual(target.innerHTML, `<p>3</p> <input type="number" />`);
+
+		input.focus();
+		input.value = '1';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		await tick();
+
+		assert.equal(input.value, '2');
+		assert.htmlEqual(target.innerHTML, `<p>2</p> <input type="number" />`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/main.svelte
new file mode 100644
index 000000000000..763ce6ebf073
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	let value = $state(0)
+	const min = 2
+	const max = 5
+
+	$effect(() => {
+		setValue()
+	})
+
+	function setValue() {
+		if (value < min) {
+			value = min
+		}
+		if (value > max) {
+			value = max
+		}
+	}
+</script>
+
+<svelte:boundary>
+	<p>{await value}</p>
+	<input type="number" bind:value />
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>
diff --git a/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/_config.js b/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/_config.js
new file mode 100644
index 000000000000..7a56c79d7176
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/_config.js
@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const [input] = target.querySelectorAll('input');
+
+		input.focus();
+		input.value = '3';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		flushSync();
+
+		assert.equal(input.value, '3');
+		assert.htmlEqual(target.innerHTML, `<p>3</p> <input type="number" />`);
+
+		input.focus();
+		input.value = '1';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		flushSync();
+
+		assert.equal(input.value, '2');
+		assert.htmlEqual(target.innerHTML, `<p>2</p> <input type="number" />`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/main.svelte b/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/main.svelte
new file mode 100644
index 000000000000..b0597c223b99
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/main.svelte
@@ -0,0 +1,21 @@
+<script>
+	let value = $state(0)
+	const min = 2
+	const max = 5
+
+	$effect(() => {
+		setValue()
+	})
+
+	function setValue() {
+		if (value < min) {
+			value = min
+		}
+		if (value > max) {
+			value = max
+		}
+	}
+</script>
+
+<p>{value}</p>
+<input type="number" bind:value />

From 9e54e9ae20205182cdcc13e26db9056353c79a15 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Jul 2025 22:56:16 +0200
Subject: [PATCH 05/19] this fixes the last test somehow

---
 .../svelte/src/internal/client/reactivity/batch.js    | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 2c959855002c..f230159978a8 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -472,11 +472,14 @@ export function flushSync(fn) {
 					batch.flush();
 				}
 
-				// this would be reset in `batch.flush_effects()` but since we are early returning here,
-				// we need to reset it here as well in case the first time there's 0 queued root effects
-				last_scheduled_effect = null;
+				// TODO this feels wrong
+				if (queued_root_effects.length === 0) {
+					// this would be reset in `batch.flush_effects()` but since we are early returning here,
+					// we need to reset it here as well in case the first time there's 0 queued root effects
+					last_scheduled_effect = null;
 
-				return /** @type {T} */ (result);
+					return /** @type {T} */ (result);
+				}
 			}
 
 			batch.flush_effects();

From 23b44ced5225bbcf4a3133c5c3f8514b77f4ffd5 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 16:57:22 -0400
Subject: [PATCH 06/19] fix #16477

---
 packages/svelte/src/internal/client/reactivity/batch.js   | 8 ++++++--
 .../samples/1000-reading-derived-effects/Component.svelte | 7 +++++++
 .../samples/1000-reading-derived-effects/_config.js       | 5 +++++
 .../samples/1000-reading-derived-effects/main.svelte      | 8 ++++++++
 4 files changed, 26 insertions(+), 2 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/Component.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 2c959855002c..ea4698073c64 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -563,7 +563,7 @@ function flush_queued_effects(effects) {
 		var effect = effects[i++];
 
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
-			var wv = write_version;
+			var n = current_batch ? current_batch.current.size : 0;
 
 			update_effect(effect);
 
@@ -586,7 +586,11 @@ function flush_queued_effects(effects) {
 
 			// if state is written in a user effect, abort and re-schedule, lest we run
 			// effects that should be removed as a result of the state change
-			if (write_version > wv && (effect.f & USER_EFFECT) !== 0) {
+			if (
+				current_batch !== null &&
+				current_batch.current.size > n &&
+				(effect.f & USER_EFFECT) !== 0
+			) {
 				break;
 			}
 		}
diff --git a/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/Component.svelte b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/Component.svelte
new file mode 100644
index 000000000000..7a54323cb97f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/Component.svelte
@@ -0,0 +1,7 @@
+<script>
+	const der = $derived(false);
+
+	$effect(() => {
+		der
+	});
+</script>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/_config.js b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/_config.js
new file mode 100644
index 000000000000..2e4a27cf0912
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/_config.js
@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	async test() {}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte
new file mode 100644
index 000000000000..258918e903bd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte
@@ -0,0 +1,8 @@
+<script>
+	import Component from './Component.svelte'	
+	const arr = Array.from({length: 10001});
+</script>
+
+{#each arr}
+	<Component />
+{/each}

From 09357ae9529c9b7dfa6d64d45132b76cd6201eca Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 18:25:07 -0400
Subject: [PATCH 07/19] typo

---
 .../samples/1000-reading-derived-effects/main.svelte            | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte
index 258918e903bd..bd326edfb92a 100644
--- a/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/1000-reading-derived-effects/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	import Component from './Component.svelte'	
+	import Component from './Component.svelte';
 	const arr = Array.from({length: 10001});
 </script>
 

From a77ee84a1b2254e1e6357a19a53c982d40e121f7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 18:48:52 -0400
Subject: [PATCH 08/19] copy over changeset from #16477

---
 .changeset/thick-mice-kick.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/thick-mice-kick.md

diff --git a/.changeset/thick-mice-kick.md b/.changeset/thick-mice-kick.md
new file mode 100644
index 000000000000..eec55b77eee4
--- /dev/null
+++ b/.changeset/thick-mice-kick.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: exclude derived writes from effect abort and rescheduling

From 72f8bfe73af57a23f59148df5f285996f616e6ec Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 18:49:45 -0400
Subject: [PATCH 09/19] copy over changeset from #16464

---
 .changeset/grumpy-boats-beg.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/grumpy-boats-beg.md

diff --git a/.changeset/grumpy-boats-beg.md b/.changeset/grumpy-boats-beg.md
new file mode 100644
index 000000000000..f677743defa5
--- /dev/null
+++ b/.changeset/grumpy-boats-beg.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep input in sync when binding updated via effect

From 0ead78284bc39ea6625249b8323c2086dcfa59e2 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 18:51:00 -0400
Subject: [PATCH 10/19] changeset

---
 .changeset/shiny-walls-fix.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/shiny-walls-fix.md

diff --git a/.changeset/shiny-walls-fix.md b/.changeset/shiny-walls-fix.md
new file mode 100644
index 000000000000..91ed548728a3
--- /dev/null
+++ b/.changeset/shiny-walls-fix.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent infinite async loop

From f92dfc158aeb23ec8a07be75f4359a8342d392cb Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 19:00:55 -0400
Subject: [PATCH 11/19] dedupe

---
 packages/svelte/src/internal/client/reactivity/sources.js | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 41fda858a52b..00c788596d2f 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -324,14 +324,16 @@ export function mark_reactions(signal, status, schedule_async = true) {
 			continue;
 		}
 
+		var should_schedule = (flags & DIRTY) === 0 && (schedule_async || (flags & ASYNC) === 0);
+
 		// don't set a DIRTY reaction to MAYBE_DIRTY
-		if ((flags & DIRTY) === 0 && (schedule_async || (flags & ASYNC) === 0)) {
+		if (should_schedule) {
 			set_signal_status(reaction, status);
 		}
 
 		if ((flags & DERIVED) !== 0) {
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
-		} else if ((flags & DIRTY) === 0 && (schedule_async || (flags & ASYNC) === 0)) {
+		} else if (should_schedule) {
 			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}

From a6d464478a1f18009bfb42ee356870dae5b66eca Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 19:04:38 -0400
Subject: [PATCH 12/19] move flushing_sync check inside Batch.ensure

---
 packages/svelte/src/internal/client/reactivity/batch.js   | 8 ++++----
 packages/svelte/src/internal/client/reactivity/sources.js | 4 ++--
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index edbebbd3a261..c5371ad7cc9c 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -70,7 +70,7 @@ let last_scheduled_effect = null;
 
 let is_flushing = false;
 
-export let flushing_sync = false;
+let flushing_sync = false;
 
 export class Batch {
 	/**
@@ -407,12 +407,12 @@ export class Batch {
 		return (this.#deferred ??= deferred()).promise;
 	}
 
-	static ensure(autoflush = true) {
+	static ensure() {
 		if (current_batch === null) {
 			const batch = (current_batch = new Batch());
 			batches.add(current_batch);
 
-			if (autoflush) {
+			if (!flushing_sync) {
 				Batch.enqueue(() => {
 					if (current_batch !== batch) {
 						// a flushSync happened in the meantime
@@ -455,7 +455,7 @@ export function flushSync(fn) {
 	try {
 		var result;
 
-		const batch = Batch.ensure(false);
+		const batch = Batch.ensure();
 
 		if (fn) {
 			batch.flush_effects();
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 00c788596d2f..3b28c8fdceeb 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -33,7 +33,7 @@ import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { get_stack, tag_proxy } from '../dev/tracing.js';
 import { component_context, is_runes } from '../context.js';
-import { Batch, flushing_sync, schedule_effect } from './batch.js';
+import { Batch, schedule_effect } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 
@@ -179,7 +179,7 @@ export function internal_set(source, value) {
 
 		source.v = value;
 
-		const batch = Batch.ensure(!flushing_sync);
+		var batch = Batch.ensure();
 		batch.capture(source, old_value);
 
 		if (DEV) {

From f68635595c993a4198cafae8b9ad173f6a33b2d1 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 19:04:58 -0400
Subject: [PATCH 13/19] unused

---
 packages/svelte/src/internal/client/reactivity/batch.js | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index c5371ad7cc9c..eeb40c761faf 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -21,8 +21,7 @@ import {
 	is_updating_effect,
 	set_is_updating_effect,
 	set_signal_status,
-	update_effect,
-	write_version
+	update_effect
 } from '../runtime.js';
 import * as e from '../errors.js';
 import { flush_tasks } from '../dom/task.js';

From a98c75ea31a8c946ff50151e81d0f6a65e08bac7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 19:05:20 -0400
Subject: [PATCH 14/19] flushing_sync -> is_flushing_sync

---
 .../svelte/src/internal/client/reactivity/batch.js     | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index eeb40c761faf..a08440d8c948 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -69,7 +69,7 @@ let last_scheduled_effect = null;
 
 let is_flushing = false;
 
-let flushing_sync = false;
+let is_flushing_sync = false;
 
 export class Batch {
 	/**
@@ -411,7 +411,7 @@ export class Batch {
 			const batch = (current_batch = new Batch());
 			batches.add(current_batch);
 
-			if (!flushing_sync) {
+			if (!is_flushing_sync) {
 				Batch.enqueue(() => {
 					if (current_batch !== batch) {
 						// a flushSync happened in the meantime
@@ -448,8 +448,8 @@ export function flushSync(fn) {
 		e.flush_sync_in_effect();
 	}
 
-	var prev_flushing_sync = flushing_sync;
-	flushing_sync = true;
+	var prev_flushing_sync = is_flushing_sync;
+	is_flushing_sync = true;
 
 	try {
 		var result;
@@ -484,7 +484,7 @@ export function flushSync(fn) {
 			batch.flush_effects();
 		}
 	} finally {
-		flushing_sync = prev_flushing_sync;
+		is_flushing_sync = prev_flushing_sync;
 	}
 }
 

From 457b8495307f35a51d97bbcffffa178dcfc52b7c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 19:07:33 -0400
Subject: [PATCH 15/19] remove flush_effects method

---
 .../src/internal/client/reactivity/batch.js     | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index a08440d8c948..0bb2970e8b4f 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -341,13 +341,13 @@ export class Batch {
 
 	flush() {
 		if (queued_root_effects.length > 0) {
-			this.flush_effects();
+			flush_effects();
 		} else {
 			this.#commit();
 		}
 
 		if (current_batch !== this) {
-			// this can happen if a `flushSync` occurred during `this.flush_effects()`,
+			// this can happen if a `flushSync` occurred during `flush_effects()`,
 			// which is permitted in legacy mode despite being a terrible idea
 			return;
 		}
@@ -359,10 +359,6 @@ export class Batch {
 		this.deactivate();
 	}
 
-	flush_effects() {
-		flush_effects();
-	}
-
 	/**
 	 * Append and remove branches to/from the DOM
 	 */
@@ -454,11 +450,10 @@ export function flushSync(fn) {
 	try {
 		var result;
 
-		const batch = Batch.ensure();
+		var batch = Batch.ensure();
 
 		if (fn) {
-			batch.flush_effects();
-
+			flush_effects();
 			result = fn();
 		}
 
@@ -473,7 +468,7 @@ export function flushSync(fn) {
 
 				// TODO this feels wrong
 				if (queued_root_effects.length === 0) {
-					// this would be reset in `batch.flush_effects()` but since we are early returning here,
+					// this would be reset in `flush_effects()` but since we are early returning here,
 					// we need to reset it here as well in case the first time there's 0 queued root effects
 					last_scheduled_effect = null;
 
@@ -481,7 +476,7 @@ export function flushSync(fn) {
 				}
 			}
 
-			batch.flush_effects();
+			flush_effects();
 		}
 	} finally {
 		is_flushing_sync = prev_flushing_sync;

From f1f34f7382c05eb232176a82a4cc441ce7cf5166 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 20:00:21 -0400
Subject: [PATCH 16/19] dedupe declaration

---
 packages/svelte/src/internal/client/reactivity/batch.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 0bb2970e8b4f..24da6ef008e5 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -489,10 +489,11 @@ function flush_effects() {
 
 	try {
 		var flush_count = 0;
-		var batch = /** @type {Batch} */ (current_batch);
 		set_is_updating_effect(true);
 
 		while (queued_root_effects.length > 0) {
+			var batch = /** @type {Batch} */ (current_batch);
+
 			if (flush_count++ > 1000) {
 				if (DEV) {
 					var updates = new Map();
@@ -519,7 +520,6 @@ function flush_effects() {
 				infinite_loop_guard();
 			}
 
-			batch = /** @type {Batch} */ (current_batch);
 			batch.process(queued_root_effects);
 			old_values.clear();
 		}

From 10b2d64e0cafbcc3d5831e6151a3b904db9c3507 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 20:15:49 -0400
Subject: [PATCH 17/19] tweak

---
 packages/svelte/src/internal/client/reactivity/batch.js | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 24da6ef008e5..b2d9f54329c0 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -450,8 +450,6 @@ export function flushSync(fn) {
 	try {
 		var result;
 
-		var batch = Batch.ensure();
-
 		if (fn) {
 			flush_effects();
 			result = fn();
@@ -461,10 +459,7 @@ export function flushSync(fn) {
 			flush_tasks();
 
 			if (queued_root_effects.length === 0) {
-				// TODO this might need adjustment
-				if (batch === current_batch) {
-					batch.flush();
-				}
+				current_batch?.flush();
 
 				// TODO this feels wrong
 				if (queued_root_effects.length === 0) {
@@ -492,7 +487,7 @@ function flush_effects() {
 		set_is_updating_effect(true);
 
 		while (queued_root_effects.length > 0) {
-			var batch = /** @type {Batch} */ (current_batch);
+			var batch = Batch.ensure();
 
 			if (flush_count++ > 1000) {
 				if (DEV) {

From 6c2f0f92b034505ebd0893fcd3fa08a43ff7c508 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 20:16:28 -0400
Subject: [PATCH 18/19] tweak

---
 packages/svelte/src/internal/client/reactivity/batch.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index b2d9f54329c0..ddcf64d1a868 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -444,7 +444,7 @@ export function flushSync(fn) {
 		e.flush_sync_in_effect();
 	}
 
-	var prev_flushing_sync = is_flushing_sync;
+	var was_flushing_sync = is_flushing_sync;
 	is_flushing_sync = true;
 
 	try {
@@ -474,7 +474,7 @@ export function flushSync(fn) {
 			flush_effects();
 		}
 	} finally {
-		is_flushing_sync = prev_flushing_sync;
+		is_flushing_sync = was_flushing_sync;
 	}
 }
 

From 82e431fb771e098fcf1242a8d09f2e9ea00f42f8 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 22 Jul 2025 20:33:53 -0400
Subject: [PATCH 19/19] =?UTF-8?q?update=20comment=20=E2=80=94=20it=20=5Fdo?=
 =?UTF-8?q?es=5F=20feel=20slightly=20wrong,=20but=20no=20wronger=20than=20?=
 =?UTF-8?q?the=20rest=20of=20this=20cursed=20function?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 packages/svelte/src/internal/client/reactivity/batch.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index ddcf64d1a868..ce413fa1e186 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -461,7 +461,7 @@ export function flushSync(fn) {
 			if (queued_root_effects.length === 0) {
 				current_batch?.flush();
 
-				// TODO this feels wrong
+				// we need to check again, in case we just updated an `$effect.pending()`
 				if (queued_root_effects.length === 0) {
 					// this would be reset in `flush_effects()` but since we are early returning here,
 					// we need to reset it here as well in case the first time there's 0 queued root effects