From 0c5bfad855833458d1690c511f462235313fea80 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 16:54:07 -0500
Subject: [PATCH 01/23] this appears to be unnecessary

---
 packages/svelte/src/internal/client/runtime.js | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 8c98f948f3f0..b5cee5cd9eb7 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -726,9 +726,7 @@ function flush_queued_effects(effects) {
 
 function process_deferred() {
 	is_micro_task_queued = false;
-	if (flush_count > 1001) {
-		return;
-	}
+
 	const previous_queued_root_effects = queued_root_effects;
 	queued_root_effects = [];
 	flush_queued_root_effects(previous_queued_root_effects);

From 00b46458872dde6f3e3a0015b45d8ccc40e96644 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 17:10:13 -0500
Subject: [PATCH 02/23] DRY out

---
 packages/svelte/src/internal/client/runtime.js | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index b5cee5cd9eb7..cc8c26bb8c85 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -661,6 +661,8 @@ function infinite_loop_guard() {
  * @returns {void}
  */
 function flush_queued_root_effects(root_effects) {
+	queued_root_effects = [];
+
 	var length = root_effects.length;
 	if (length === 0) {
 		return;
@@ -727,9 +729,7 @@ function flush_queued_effects(effects) {
 function process_deferred() {
 	is_micro_task_queued = false;
 
-	const previous_queued_root_effects = queued_root_effects;
-	queued_root_effects = [];
-	flush_queued_root_effects(previous_queued_root_effects);
+	flush_queued_root_effects(queued_root_effects);
 
 	if (!is_micro_task_queued) {
 		flush_count = 0;
@@ -857,10 +857,9 @@ export function flush_sync(fn) {
 		infinite_loop_guard();
 
 		scheduler_mode = FLUSH_SYNC;
-		queued_root_effects = [];
 		is_micro_task_queued = false;
 
-		flush_queued_root_effects(previous_queued_root_effects);
+		flush_queued_root_effects(queued_root_effects);
 
 		var result = fn?.();
 

From 7d6d94c594329a70e04346af295841553196d026 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 17:11:20 -0500
Subject: [PATCH 03/23] this doesnt appear to do anything useful

---
 packages/svelte/src/internal/client/runtime.js | 2 --
 1 file changed, 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index cc8c26bb8c85..86438df527a0 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -851,7 +851,6 @@ function process_effects(effect) {
  */
 export function flush_sync(fn) {
 	var previous_scheduler_mode = scheduler_mode;
-	var previous_queued_root_effects = queued_root_effects;
 
 	try {
 		infinite_loop_guard();
@@ -877,7 +876,6 @@ export function flush_sync(fn) {
 		return result;
 	} finally {
 		scheduler_mode = previous_scheduler_mode;
-		queued_root_effects = previous_queued_root_effects;
 	}
 }
 

From 02a7cf887f341eeb7a1e189b6ad4d61420150f4b Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:29:31 -0500
Subject: [PATCH 04/23] simplify

---
 .../svelte/src/internal/client/runtime.js     | 57 +++++++++----------
 .../samples/animation-css/_config.js          |  2 +
 .../samples/animation-js-easing/_config.js    |  2 +
 .../samples/animation-js/_config.js           |  5 ++
 .../dynamic-element-animation/_config.js      |  2 +
 5 files changed, 37 insertions(+), 31 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 86438df527a0..49bd9253dadd 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -9,7 +9,6 @@ import {
 } from './reactivity/effects.js';
 import {
 	EFFECT,
-	RENDER_EFFECT,
 	DIRTY,
 	MAYBE_DIRTY,
 	CLEAN,
@@ -41,15 +40,11 @@ import {
 } from './context.js';
 import { is_firefox } from './dom/operations.js';
 
-const FLUSH_MICROTASK = 0;
-const FLUSH_SYNC = 1;
 // Used for DEV time error handling
 /** @param {WeakSet<Error>} value */
 const handled_errors = new WeakSet();
 export let is_throwing_error = false;
 
-// Used for controlling the flush of effects.
-let scheduler_mode = FLUSH_MICROTASK;
 // Used for handling scheduling
 let is_micro_task_queued = false;
 
@@ -672,6 +667,8 @@ function flush_queued_root_effects(root_effects) {
 	var previously_flushing_effect = is_flushing_effect;
 	is_flushing_effect = true;
 
+	is_flushing = true;
+
 	try {
 		for (var i = 0; i < length; i++) {
 			var effect = root_effects[i];
@@ -683,8 +680,13 @@ function flush_queued_root_effects(root_effects) {
 			var collected_effects = process_effects(effect);
 			flush_queued_effects(collected_effects);
 		}
+
+		if (queued_root_effects.length > 0) {
+			flush_queued_root_effects(queued_root_effects);
+		}
 	} finally {
 		is_flushing_effect = previously_flushing_effect;
+		is_flushing = false;
 	}
 }
 
@@ -740,16 +742,16 @@ function process_deferred() {
 	}
 }
 
+let is_flushing = false;
+
 /**
  * @param {Effect} signal
  * @returns {void}
  */
 export function schedule_effect(signal) {
-	if (scheduler_mode === FLUSH_MICROTASK) {
-		if (!is_micro_task_queued) {
-			is_micro_task_queued = true;
-			queueMicrotask(process_deferred);
-		}
+	if (!is_micro_task_queued && !is_flushing) {
+		is_micro_task_queued = true;
+		queueMicrotask(process_deferred);
 	}
 
 	last_scheduled_effect = signal;
@@ -850,33 +852,26 @@ function process_effects(effect) {
  * @returns {any}
  */
 export function flush_sync(fn) {
-	var previous_scheduler_mode = scheduler_mode;
-
-	try {
-		infinite_loop_guard();
+	infinite_loop_guard();
 
-		scheduler_mode = FLUSH_SYNC;
-		is_micro_task_queued = false;
+	is_micro_task_queued = false;
 
-		flush_queued_root_effects(queued_root_effects);
+	flush_queued_root_effects(queued_root_effects);
 
-		var result = fn?.();
+	var result = fn?.();
 
-		flush_tasks();
-		if (queued_root_effects.length > 0) {
-			flush_sync();
-		}
-
-		flush_count = 0;
-		last_scheduled_effect = null;
-		if (DEV) {
-			dev_effect_stack = [];
-		}
+	flush_tasks();
+	if (queued_root_effects.length > 0) {
+		flush_sync();
+	}
 
-		return result;
-	} finally {
-		scheduler_mode = previous_scheduler_mode;
+	flush_count = 0;
+	last_scheduled_effect = null;
+	if (DEV) {
+		dev_effect_stack = [];
 	}
+
+	return result;
 }
 
 /**
diff --git a/packages/svelte/tests/runtime-legacy/samples/animation-css/_config.js b/packages/svelte/tests/runtime-legacy/samples/animation-css/_config.js
index b6b601a96b1d..b6bd818e65db 100644
--- a/packages/svelte/tests/runtime-legacy/samples/animation-css/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/animation-css/_config.js
@@ -47,6 +47,8 @@ export default test({
 			{ id: 1, name: 'a' }
 		];
 
+		raf.tick(0);
+
 		divs = target.querySelectorAll('div');
 		assert.ok(divs[0].getAnimations().length > 0);
 		assert.equal(divs[1].getAnimations().length, 0);
diff --git a/packages/svelte/tests/runtime-legacy/samples/animation-js-easing/_config.js b/packages/svelte/tests/runtime-legacy/samples/animation-js-easing/_config.js
index 5b7ed1c73209..f4a3554b29f9 100644
--- a/packages/svelte/tests/runtime-legacy/samples/animation-js-easing/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/animation-js-easing/_config.js
@@ -46,6 +46,8 @@ export default test({
 			{ id: 1, name: 'a' }
 		];
 
+		raf.tick(0);
+
 		divs = document.querySelectorAll('div');
 		assert.equal(divs[0].dy, 120);
 		assert.equal(divs[4].dy, -120);
diff --git a/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js b/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
index 3606f7d17b77..1b3390e55f97 100644
--- a/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
@@ -1,4 +1,5 @@
 // @ts-nocheck
+import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -46,6 +47,8 @@ export default test({
 			{ id: 1, name: 'a' }
 		];
 
+		raf.tick(0);
+
 		divs = document.querySelectorAll('div');
 		assert.equal(divs[0].dy, 120);
 		assert.equal(divs[4].dy, -120);
@@ -66,6 +69,8 @@ export default test({
 			{ id: 5, name: 'e' }
 		];
 
+		raf.tick(100);
+
 		divs = document.querySelectorAll('div');
 
 		assert.equal(divs[0].dy, 120);
diff --git a/packages/svelte/tests/runtime-legacy/samples/dynamic-element-animation/_config.js b/packages/svelte/tests/runtime-legacy/samples/dynamic-element-animation/_config.js
index 3d127f1375e7..05c2dc73048a 100644
--- a/packages/svelte/tests/runtime-legacy/samples/dynamic-element-animation/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/dynamic-element-animation/_config.js
@@ -50,6 +50,8 @@ export default test({
 			{ id: 1, name: 'a' }
 		];
 
+		raf.tick(0);
+
 		divs = target.querySelectorAll('div');
 		assert.equal(divs[0].style.transform, 'translate(0px, 120px)');
 		assert.equal(divs[1].style.transform, '');

From 75f0adf3ae64e5ef2f83bb17eb5a7997a13e245c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:35:47 -0500
Subject: [PATCH 05/23] remove unused if block

---
 packages/svelte/src/internal/client/runtime.js | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 49bd9253dadd..37cbb650d60d 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -51,7 +51,7 @@ let is_micro_task_queued = false;
 /** @type {Effect | null} */
 let last_scheduled_effect = null;
 
-export let is_flushing_effect = false;
+export let is_flushing_effect = false; // TODO do we still need this?
 export let is_destroying_effect = false;
 
 /** @param {boolean} value */
@@ -733,12 +733,11 @@ function process_deferred() {
 
 	flush_queued_root_effects(queued_root_effects);
 
-	if (!is_micro_task_queued) {
-		flush_count = 0;
-		last_scheduled_effect = null;
-		if (DEV) {
-			dev_effect_stack = [];
-		}
+	flush_count = 0;
+	last_scheduled_effect = null;
+
+	if (DEV) {
+		dev_effect_stack = [];
 	}
 }
 

From a2e3fb90e2c9df19166df6851fbee5863c512a4a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:40:28 -0500
Subject: [PATCH 06/23] simplify, make non-recursive

---
 .../svelte/src/internal/client/runtime.js     | 89 +++++++++----------
 1 file changed, 42 insertions(+), 47 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 37cbb650d60d..4fab73b587b4 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -615,40 +615,36 @@ function log_effect_stack() {
 }
 
 function infinite_loop_guard() {
-	if (flush_count > 1000) {
-		flush_count = 0;
-		try {
-			e.effect_update_depth_exceeded();
-		} catch (error) {
+	try {
+		e.effect_update_depth_exceeded();
+	} catch (error) {
+		if (DEV) {
+			// stack is garbage, ignore. Instead add a console.error message.
+			define_property(error, 'stack', {
+				value: ''
+			});
+		}
+		// Try and handle the error so it can be caught at a boundary, that's
+		// if there's an effect available from when it was last scheduled
+		if (last_scheduled_effect !== null) {
 			if (DEV) {
-				// stack is garbage, ignore. Instead add a console.error message.
-				define_property(error, 'stack', {
-					value: ''
-				});
-			}
-			// Try and handle the error so it can be caught at a boundary, that's
-			// if there's an effect available from when it was last scheduled
-			if (last_scheduled_effect !== null) {
-				if (DEV) {
-					try {
-						handle_error(error, last_scheduled_effect, null, null);
-					} catch (e) {
-						// Only log the effect stack if the error is re-thrown
-						log_effect_stack();
-						throw e;
-					}
-				} else {
+				try {
 					handle_error(error, last_scheduled_effect, null, null);
-				}
-			} else {
-				if (DEV) {
+				} catch (e) {
+					// Only log the effect stack if the error is re-thrown
 					log_effect_stack();
+					throw e;
 				}
-				throw error;
+			} else {
+				handle_error(error, last_scheduled_effect, null, null);
+			}
+		} else {
+			if (DEV) {
+				log_effect_stack();
 			}
+			throw error;
 		}
 	}
-	flush_count++;
 }
 
 /**
@@ -656,33 +652,34 @@ function infinite_loop_guard() {
  * @returns {void}
  */
 function flush_queued_root_effects(root_effects) {
-	queued_root_effects = [];
-
-	var length = root_effects.length;
-	if (length === 0) {
-		return;
-	}
-	infinite_loop_guard();
-
 	var previously_flushing_effect = is_flushing_effect;
 	is_flushing_effect = true;
-
 	is_flushing = true;
 
 	try {
-		for (var i = 0; i < length; i++) {
-			var effect = root_effects[i];
+		var length = queued_root_effects.length;
+		var flush_count = 0;
 
-			if ((effect.f & CLEAN) === 0) {
-				effect.f ^= CLEAN;
+		while (queued_root_effects.length > 0) {
+			if (flush_count++ > 1000) {
+				infinite_loop_guard();
 			}
 
-			var collected_effects = process_effects(effect);
-			flush_queued_effects(collected_effects);
-		}
+			var root_effects = queued_root_effects;
+			var length = root_effects.length;
+
+			queued_root_effects = [];
 
-		if (queued_root_effects.length > 0) {
-			flush_queued_root_effects(queued_root_effects);
+			for (var i = 0; i < length; i++) {
+				var effect = root_effects[i];
+
+				if ((effect.f & CLEAN) === 0) {
+					effect.f ^= CLEAN;
+				}
+
+				var collected_effects = process_effects(effect);
+				flush_queued_effects(collected_effects);
+			}
 		}
 	} finally {
 		is_flushing_effect = previously_flushing_effect;
@@ -851,8 +848,6 @@ function process_effects(effect) {
  * @returns {any}
  */
 export function flush_sync(fn) {
-	infinite_loop_guard();
-
 	is_micro_task_queued = false;
 
 	flush_queued_root_effects(queued_root_effects);

From 3db7b311f49604d6b9e289f72db24454f5ac99ee Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:41:39 -0500
Subject: [PATCH 07/23] unused

---
 packages/svelte/src/internal/client/runtime.js | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 4fab73b587b4..3e96ea403994 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -69,7 +69,6 @@ export function set_is_destroying_effect(value) {
 /** @type {Effect[]} */
 let queued_root_effects = [];
 
-let flush_count = 0;
 /** @type {Effect[]} Stack of effects, dev only */
 let dev_effect_stack = [];
 // Handle signal reactivity tree dependencies and reactions
@@ -730,7 +729,6 @@ function process_deferred() {
 
 	flush_queued_root_effects(queued_root_effects);
 
-	flush_count = 0;
 	last_scheduled_effect = null;
 
 	if (DEV) {
@@ -859,7 +857,6 @@ export function flush_sync(fn) {
 		flush_sync();
 	}
 
-	flush_count = 0;
 	last_scheduled_effect = null;
 	if (DEV) {
 		dev_effect_stack = [];

From 8f66f9df11e85cf26290c6b7a172b4d570f26fa9 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:44:12 -0500
Subject: [PATCH 08/23] DRY

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

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 3e96ea403994..c8259c929e1b 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -652,6 +652,8 @@ function infinite_loop_guard() {
  */
 function flush_queued_root_effects(root_effects) {
 	var previously_flushing_effect = is_flushing_effect;
+
+	is_micro_task_queued = false;
 	is_flushing_effect = true;
 	is_flushing = true;
 
@@ -725,8 +727,6 @@ function flush_queued_effects(effects) {
 }
 
 function process_deferred() {
-	is_micro_task_queued = false;
-
 	flush_queued_root_effects(queued_root_effects);
 
 	last_scheduled_effect = null;
@@ -846,8 +846,6 @@ function process_effects(effect) {
  * @returns {any}
  */
 export function flush_sync(fn) {
-	is_micro_task_queued = false;
-
 	flush_queued_root_effects(queued_root_effects);
 
 	var result = fn?.();

From a45322659721c8ea780c2cd433ccc688a32f4285 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 21:46:35 -0500
Subject: [PATCH 09/23] simplify

---
 .../svelte/src/internal/client/runtime.js     | 30 +++++--------------
 1 file changed, 8 insertions(+), 22 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index c8259c929e1b..f490de45b1c3 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -646,11 +646,7 @@ function infinite_loop_guard() {
 	}
 }
 
-/**
- * @param {Array<Effect>} root_effects
- * @returns {void}
- */
-function flush_queued_root_effects(root_effects) {
+function flush_queued_root_effects() {
 	var previously_flushing_effect = is_flushing_effect;
 
 	is_micro_task_queued = false;
@@ -685,6 +681,11 @@ function flush_queued_root_effects(root_effects) {
 	} finally {
 		is_flushing_effect = previously_flushing_effect;
 		is_flushing = false;
+
+		last_scheduled_effect = null;
+		if (DEV) {
+			dev_effect_stack = [];
+		}
 	}
 }
 
@@ -726,16 +727,6 @@ function flush_queued_effects(effects) {
 	}
 }
 
-function process_deferred() {
-	flush_queued_root_effects(queued_root_effects);
-
-	last_scheduled_effect = null;
-
-	if (DEV) {
-		dev_effect_stack = [];
-	}
-}
-
 let is_flushing = false;
 
 /**
@@ -745,7 +736,7 @@ let is_flushing = false;
 export function schedule_effect(signal) {
 	if (!is_micro_task_queued && !is_flushing) {
 		is_micro_task_queued = true;
-		queueMicrotask(process_deferred);
+		queueMicrotask(flush_queued_root_effects);
 	}
 
 	last_scheduled_effect = signal;
@@ -846,7 +837,7 @@ function process_effects(effect) {
  * @returns {any}
  */
 export function flush_sync(fn) {
-	flush_queued_root_effects(queued_root_effects);
+	flush_queued_root_effects();
 
 	var result = fn?.();
 
@@ -855,11 +846,6 @@ export function flush_sync(fn) {
 		flush_sync();
 	}
 
-	last_scheduled_effect = null;
-	if (DEV) {
-		dev_effect_stack = [];
-	}
-
 	return result;
 }
 

From e42172add4c608d637e65e050ceb0a321f458813 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:01:03 -0500
Subject: [PATCH 10/23] tidy up

---
 .../svelte/src/internal/client/dom/task.js    | 47 +++++++++----------
 .../svelte/src/internal/client/runtime.js     | 12 ++---
 2 files changed, 27 insertions(+), 32 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/task.js b/packages/svelte/src/internal/client/dom/task.js
index acb5a5b117f0..fe37d4828f7a 100644
--- a/packages/svelte/src/internal/client/dom/task.js
+++ b/packages/svelte/src/internal/client/dom/task.js
@@ -6,25 +6,21 @@ export const request_idle_callback =
 		? (/** @type {() => void} */ cb) => setTimeout(cb, 1)
 		: requestIdleCallback;
 
-let is_micro_task_queued = false;
-let is_idle_task_queued = false;
-
 /** @type {Array<() => void>} */
-let current_queued_micro_tasks = [];
+let micro_tasks = [];
+
 /** @type {Array<() => void>} */
-let current_queued_idle_tasks = [];
+let idle_tasks = [];
 
-function process_micro_tasks() {
-	is_micro_task_queued = false;
-	const tasks = current_queued_micro_tasks.slice();
-	current_queued_micro_tasks = [];
+function run_micro_tasks() {
+	var tasks = micro_tasks;
+	micro_tasks = [];
 	run_all(tasks);
 }
 
-function process_idle_tasks() {
-	is_idle_task_queued = false;
-	const tasks = current_queued_idle_tasks.slice();
-	current_queued_idle_tasks = [];
+function run_idle_tasks() {
+	var tasks = idle_tasks;
+	idle_tasks = [];
 	run_all(tasks);
 }
 
@@ -32,32 +28,33 @@ function process_idle_tasks() {
  * @param {() => void} fn
  */
 export function queue_micro_task(fn) {
-	if (!is_micro_task_queued) {
-		is_micro_task_queued = true;
-		queueMicrotask(process_micro_tasks);
+	if (micro_tasks.length === 0) {
+		queueMicrotask(run_micro_tasks);
 	}
-	current_queued_micro_tasks.push(fn);
+
+	micro_tasks.push(fn);
 }
 
 /**
  * @param {() => void} fn
  */
 export function queue_idle_task(fn) {
-	if (!is_idle_task_queued) {
-		is_idle_task_queued = true;
-		request_idle_callback(process_idle_tasks);
+	if (idle_tasks.length === 0) {
+		request_idle_callback(run_idle_tasks);
 	}
-	current_queued_idle_tasks.push(fn);
+
+	idle_tasks.push(fn);
 }
 
 /**
  * Synchronously run any queued tasks.
  */
 export function flush_tasks() {
-	if (is_micro_task_queued) {
-		process_micro_tasks();
+	if (micro_tasks.length > 0) {
+		run_micro_tasks();
 	}
-	if (is_idle_task_queued) {
-		process_idle_tasks();
+
+	if (idle_tasks.length > 0) {
+		run_idle_tasks();
 	}
 }
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index f490de45b1c3..647512b55fb4 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -668,13 +668,13 @@ function flush_queued_root_effects() {
 			queued_root_effects = [];
 
 			for (var i = 0; i < length; i++) {
-				var effect = root_effects[i];
+				var root = root_effects[i];
 
-				if ((effect.f & CLEAN) === 0) {
-					effect.f ^= CLEAN;
+				if ((root.f & CLEAN) === 0) {
+					root.f ^= CLEAN;
 				}
 
-				var collected_effects = process_effects(effect);
+				var collected_effects = process_effects(root);
 				flush_queued_effects(collected_effects);
 			}
 		}
@@ -739,9 +739,7 @@ export function schedule_effect(signal) {
 		queueMicrotask(flush_queued_root_effects);
 	}
 
-	last_scheduled_effect = signal;
-
-	var effect = signal;
+	var effect = (last_scheduled_effect = signal);
 
 	while (effect.parent !== null) {
 		effect = effect.parent;

From 36092a57569a9124edcced9c7fc9d76fc6da04ab Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:07:02 -0500
Subject: [PATCH 11/23] simplify

---
 packages/svelte/src/internal/client/runtime.js | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 647512b55fb4..d2b1b8d7d963 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -45,8 +45,7 @@ import { is_firefox } from './dom/operations.js';
 const handled_errors = new WeakSet();
 export let is_throwing_error = false;
 
-// Used for handling scheduling
-let is_micro_task_queued = false;
+let is_flushing = false;
 
 /** @type {Effect | null} */
 let last_scheduled_effect = null;
@@ -648,10 +647,7 @@ function infinite_loop_guard() {
 
 function flush_queued_root_effects() {
 	var previously_flushing_effect = is_flushing_effect;
-
-	is_micro_task_queued = false;
 	is_flushing_effect = true;
-	is_flushing = true;
 
 	try {
 		var length = queued_root_effects.length;
@@ -727,15 +723,13 @@ function flush_queued_effects(effects) {
 	}
 }
 
-let is_flushing = false;
-
 /**
  * @param {Effect} signal
  * @returns {void}
  */
 export function schedule_effect(signal) {
-	if (!is_micro_task_queued && !is_flushing) {
-		is_micro_task_queued = true;
+	if (!is_flushing) {
+		is_flushing = true;
 		queueMicrotask(flush_queued_root_effects);
 	}
 

From 946e00dcf7f773a37ffd8a5746aa7ac36ff9fc03 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:07:59 -0500
Subject: [PATCH 12/23] changeset

---
 .changeset/mighty-dryers-smoke.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/mighty-dryers-smoke.md

diff --git a/.changeset/mighty-dryers-smoke.md b/.changeset/mighty-dryers-smoke.md
new file mode 100644
index 000000000000..c9b638aacdb2
--- /dev/null
+++ b/.changeset/mighty-dryers-smoke.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly initialise effects inside nested root

From a36e83f4e762375aee499b36bc31aa07224b5582 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:15:14 -0500
Subject: [PATCH 13/23] unused

---
 .../svelte/tests/runtime-legacy/samples/animation-js/_config.js  | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js b/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
index 1b3390e55f97..a2e17b49f869 100644
--- a/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
+++ b/packages/svelte/tests/runtime-legacy/samples/animation-js/_config.js
@@ -1,5 +1,4 @@
 // @ts-nocheck
-import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({

From 99033bdbfa6f35d5dd4baf24bad97e59f72f1e48 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:16:06 -0500
Subject: [PATCH 14/23] Revert "changeset"

This reverts commit 946e00dcf7f773a37ffd8a5746aa7ac36ff9fc03.
---
 .changeset/mighty-dryers-smoke.md | 5 -----
 1 file changed, 5 deletions(-)
 delete mode 100644 .changeset/mighty-dryers-smoke.md

diff --git a/.changeset/mighty-dryers-smoke.md b/.changeset/mighty-dryers-smoke.md
deleted file mode 100644
index c9b638aacdb2..000000000000
--- a/.changeset/mighty-dryers-smoke.md
+++ /dev/null
@@ -1,5 +0,0 @@
----
-'svelte': patch
----
-
-fix: correctly initialise effects inside nested root

From c45b60c93575f8cac4df5842714a6d7341aa8b2c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:24:37 -0500
Subject: [PATCH 15/23] make flush_sync non-recursive

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

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index d2b1b8d7d963..1b2cc4a6cdb4 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -834,8 +834,10 @@ export function flush_sync(fn) {
 	var result = fn?.();
 
 	flush_tasks();
-	if (queued_root_effects.length > 0) {
-		flush_sync();
+
+	while (queued_root_effects.length > 0) {
+		flush_queued_root_effects();
+		flush_tasks();
 	}
 
 	return result;

From 8af20fe3fc641aeb8d0a0705c5d905183c02f7e9 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:38:04 -0500
Subject: [PATCH 16/23] fix flushSync types

---
 .changeset/violet-camels-heal.md              |  5 +++
 .../3-transform/client/transform-client.js    |  2 +-
 packages/svelte/src/index-client.js           | 12 +----
 .../src/internal/client/dom/blocks/await.js   |  4 +-
 packages/svelte/src/internal/client/index.js  |  2 +-
 .../svelte/src/internal/client/runtime.js     | 17 +++----
 packages/svelte/src/legacy/legacy-client.js   |  6 +--
 packages/svelte/tests/store/test.ts           |  8 ++--
 packages/svelte/types/index.d.ts              | 45 ++++++++++---------
 9 files changed, 50 insertions(+), 51 deletions(-)
 create mode 100644 .changeset/violet-camels-heal.md

diff --git a/.changeset/violet-camels-heal.md b/.changeset/violet-camels-heal.md
new file mode 100644
index 000000000000..31e72fa33d53
--- /dev/null
+++ b/.changeset/violet-camels-heal.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: update types and inline docs for flushSync
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 2e6307a4b7a6..cf5ba285cbf3 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -312,7 +312,7 @@ export function client_component(analysis, options) {
 
 			const setter = b.set(key, [
 				b.stmt(b.call(b.id(name), b.id('$$value'))),
-				b.stmt(b.call('$.flush_sync'))
+				b.stmt(b.call('$.flush'))
 			]);
 
 			if (analysis.runes && binding.initial) {
diff --git a/packages/svelte/src/index-client.js b/packages/svelte/src/index-client.js
index ca29d5bfbe3c..efcf7b727b8d 100644
--- a/packages/svelte/src/index-client.js
+++ b/packages/svelte/src/index-client.js
@@ -1,7 +1,7 @@
 /** @import { ComponentContext, ComponentContextLegacy } from '#client' */
 /** @import { EventDispatcher } from './index.js' */
 /** @import { NotFunction } from './internal/types.js' */
-import { flush_sync, untrack } from './internal/client/runtime.js';
+import { untrack } from './internal/client/runtime.js';
 import { is_array } from './internal/shared/utils.js';
 import { user_effect } from './internal/client/index.js';
 import * as e from './internal/client/errors.js';
@@ -206,15 +206,7 @@ function init_update_callbacks(context) {
 	return (l.u ??= { a: [], b: [], m: [] });
 }
 
-/**
- * Synchronously flushes any pending state changes and those that result from it.
- * @param {() => void} [fn]
- * @returns {void}
- */
-export function flushSync(fn) {
-	flush_sync(fn);
-}
-
+export { flushSync } from './internal/client/runtime.js';
 export { getContext, getAllContexts, hasContext, setContext } from './internal/client/context.js';
 export { hydrate, mount, unmount } from './internal/client/render.js';
 export { tick, untrack } from './internal/client/runtime.js';
diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index c8c7c1c0ea77..2e3d22977914 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -3,7 +3,7 @@ import { DEV } from 'esm-env';
 import { is_promise } from '../../../shared/utils.js';
 import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
 import { internal_set, mutable_source, source } from '../../reactivity/sources.js';
-import { flush_sync, set_active_effect, set_active_reaction } from '../../runtime.js';
+import { flushSync, set_active_effect, set_active_reaction } from '../../runtime.js';
 import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
 import { queue_micro_task } from '../task.js';
 import { UNINITIALIZED } from '../../../../constants.js';
@@ -105,7 +105,7 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 
 				// without this, the DOM does not update until two ticks after the promise
 				// resolves, which is unexpected behaviour (and somewhat irksome to test)
-				flush_sync();
+				flushSync();
 			}
 		}
 	}
diff --git a/packages/svelte/src/internal/client/index.js b/packages/svelte/src/internal/client/index.js
index d78f6d452e84..f5ccb9413ba0 100644
--- a/packages/svelte/src/internal/client/index.js
+++ b/packages/svelte/src/internal/client/index.js
@@ -137,7 +137,7 @@ export {
 	get,
 	safe_get,
 	invalidate_inner_signals,
-	flush_sync,
+	flushSync as flush,
 	tick,
 	untrack,
 	exclude_from_object,
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 1b2cc4a6cdb4..cba30376ab08 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -823,12 +823,13 @@ function process_effects(effect) {
 }
 
 /**
- * Internal version of `flushSync` with the option to not flush previous effects.
- * Returns the result of the passed function, if given.
- * @param {() => any} [fn]
- * @returns {any}
+ * Synchronously flush any pending updates.
+ * Returns void if no callback is provided, otherwise returns the result of calling the callback.
+ * @template [T=void]
+ * @param {(() => T) | undefined} [fn]
+ * @returns {T extends void ? void : T}
  */
-export function flush_sync(fn) {
+export function flushSync(fn) {
 	flush_queued_root_effects();
 
 	var result = fn?.();
@@ -840,7 +841,7 @@ export function flush_sync(fn) {
 		flush_tasks();
 	}
 
-	return result;
+	return /** @type {T extends void ? void : T} */ (result);
 }
 
 /**
@@ -849,9 +850,9 @@ export function flush_sync(fn) {
  */
 export async function tick() {
 	await Promise.resolve();
-	// By calling flush_sync we guarantee that any pending state changes are applied after one tick.
+	// By calling flushSync we guarantee that any pending state changes are applied after one tick.
 	// TODO look into whether we can make flushing subsequent updates synchronously in the future.
-	flush_sync();
+	flushSync();
 }
 
 /**
diff --git a/packages/svelte/src/legacy/legacy-client.js b/packages/svelte/src/legacy/legacy-client.js
index 3a05bc04963f..bb9a5a9c039b 100644
--- a/packages/svelte/src/legacy/legacy-client.js
+++ b/packages/svelte/src/legacy/legacy-client.js
@@ -3,7 +3,7 @@ import { DIRTY, LEGACY_PROPS, MAYBE_DIRTY } from '../internal/client/constants.j
 import { user_pre_effect } from '../internal/client/reactivity/effects.js';
 import { mutable_source, set } from '../internal/client/reactivity/sources.js';
 import { hydrate, mount, unmount } from '../internal/client/render.js';
-import { active_effect, flush_sync, get, set_signal_status } from '../internal/client/runtime.js';
+import { active_effect, flushSync, get, set_signal_status } from '../internal/client/runtime.js';
 import { lifecycle_outside_component } from '../internal/shared/errors.js';
 import { define_property, is_array } from '../internal/shared/utils.js';
 import * as w from '../internal/client/warnings.js';
@@ -119,9 +119,9 @@ class Svelte4Component {
 			recover: options.recover
 		});
 
-		// We don't flush_sync for custom element wrappers or if the user doesn't want it
+		// We don't flushSync for custom element wrappers or if the user doesn't want it
 		if (!options?.props?.$$host || options.sync === false) {
-			flush_sync();
+			flushSync();
 		}
 
 		this.#events = props.$$events;
diff --git a/packages/svelte/tests/store/test.ts b/packages/svelte/tests/store/test.ts
index b23ea195d6c9..77cecca7e525 100644
--- a/packages/svelte/tests/store/test.ts
+++ b/packages/svelte/tests/store/test.ts
@@ -602,7 +602,7 @@ describe('toStore', () => {
 		assert.deepEqual(log, [0]);
 
 		set(count, 1);
-		$.flush_sync();
+		$.flushSync();
 		assert.deepEqual(log, [0, 1]);
 
 		unsubscribe();
@@ -625,7 +625,7 @@ describe('toStore', () => {
 		assert.deepEqual(log, [0]);
 
 		set(count, 1);
-		$.flush_sync();
+		$.flushSync();
 		assert.deepEqual(log, [0, 1]);
 
 		store.set(2);
@@ -654,11 +654,11 @@ describe('fromStore', () => {
 		assert.deepEqual(log, [0]);
 
 		store.set(1);
-		$.flush_sync();
+		$.flushSync();
 		assert.deepEqual(log, [0, 1]);
 
 		count.current = 2;
-		$.flush_sync();
+		$.flushSync();
 		assert.deepEqual(log, [0, 1, 2]);
 
 		assert.equal(get(store), 2);
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index 77d78477ee93..4bfb160395f1 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -408,10 +408,6 @@ declare module 'svelte' {
 	 * @deprecated Use [`$effect`](https://svelte.dev/docs/svelte/$effect) instead
 	 * */
 	export function afterUpdate(fn: () => void): void;
-	/**
-	 * Synchronously flushes any pending state changes and those that result from it.
-	 * */
-	export function flushSync(fn?: (() => void) | undefined): void;
 	/**
 	 * Create a snippet programmatically
 	 * */
@@ -421,6 +417,29 @@ declare module 'svelte' {
 	}): Snippet<Params>;
 	/** Anything except a function */
 	type NotFunction<T> = T extends Function ? never : T;
+	/**
+	 * Synchronously flush any pending updates.
+	 * Returns void if no callback is provided, otherwise returns the result of calling the callback.
+	 * */
+	export function flushSync<T = void>(fn?: (() => T) | undefined): T extends void ? void : T;
+	/**
+	 * Returns a promise that resolves once any pending state changes have been applied.
+	 * */
+	export function tick(): Promise<void>;
+	/**
+	 * When used inside a [`$derived`](https://svelte.dev/docs/svelte/$derived) or [`$effect`](https://svelte.dev/docs/svelte/$effect),
+	 * any state read inside `fn` will not be treated as a dependency.
+	 *
+	 * ```ts
+	 * $effect(() => {
+	 *   // this will run when `data` changes, but not when `time` changes
+	 *   save(data, {
+	 *     timestamp: untrack(() => time)
+	 *   });
+	 * });
+	 * ```
+	 * */
+	export function untrack<T>(fn: () => T): T;
 	/**
 	 * Retrieves the context that belongs to the closest parent component with the specified `key`.
 	 * Must be called during component initialisation.
@@ -494,24 +513,6 @@ declare module 'svelte' {
 	export function unmount(component: Record<string, any>, options?: {
 		outro?: boolean;
 	} | undefined): Promise<void>;
-	/**
-	 * Returns a promise that resolves once any pending state changes have been applied.
-	 * */
-	export function tick(): Promise<void>;
-	/**
-	 * When used inside a [`$derived`](https://svelte.dev/docs/svelte/$derived) or [`$effect`](https://svelte.dev/docs/svelte/$effect),
-	 * any state read inside `fn` will not be treated as a dependency.
-	 *
-	 * ```ts
-	 * $effect(() => {
-	 *   // this will run when `data` changes, but not when `time` changes
-	 *   save(data, {
-	 *     timestamp: untrack(() => time)
-	 *   });
-	 * });
-	 * ```
-	 * */
-	export function untrack<T>(fn: () => T): T;
 	type Getters<T> = {
 		[K in keyof T]: () => T[K];
 	};

From ccfcbfc9b30d376a3d19ec2e53501c48f1672d7c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:47:55 -0500
Subject: [PATCH 17/23] fix

---
 benchmarking/benchmarks/reactivity/kairo/kairo_avoidable.js | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_broad.js     | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_deep.js      | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_diamond.js   | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_mux.js       | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_repeated.js  | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_triangle.js  | 4 ++--
 benchmarking/benchmarks/reactivity/kairo/kairo_unstable.js  | 4 ++--
 benchmarking/benchmarks/reactivity/mol_bench.js             | 4 ++--
 9 files changed, 18 insertions(+), 18 deletions(-)

diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_avoidable.js b/benchmarking/benchmarks/reactivity/kairo/kairo_avoidable.js
index 6b058cdc3c1d..9daea6de99cb 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_avoidable.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_avoidable.js
@@ -20,12 +20,12 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			assert($.get(computed5) === 6);
 			for (let i = 0; i < 1000; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(computed5) === 6);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_broad.js b/benchmarking/benchmarks/reactivity/kairo/kairo_broad.js
index d1cde5958edf..8dc5710c87db 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_broad.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_broad.js
@@ -25,12 +25,12 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			counter = 0;
 			for (let i = 0; i < 50; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(last) === i + 50);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_deep.js b/benchmarking/benchmarks/reactivity/kairo/kairo_deep.js
index 149457ede156..8690c85f864a 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_deep.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_deep.js
@@ -25,12 +25,12 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			counter = 0;
 			for (let i = 0; i < iter; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(current) === len + i);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_diamond.js b/benchmarking/benchmarks/reactivity/kairo/kairo_diamond.js
index 958a1bcd7890..bf4e07ee8962 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_diamond.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_diamond.js
@@ -28,13 +28,13 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			assert($.get(sum) === 2 * width);
 			counter = 0;
 			for (let i = 0; i < 500; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(sum) === (i + 1) * width);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_mux.js b/benchmarking/benchmarks/reactivity/kairo/kairo_mux.js
index b645051c09bf..fc252a27b5f8 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_mux.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_mux.js
@@ -22,13 +22,13 @@ function setup() {
 		destroy,
 		run() {
 			for (let i = 0; i < 10; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(heads[i], i);
 				});
 				assert($.get(splited[i]) === i + 1);
 			}
 			for (let i = 0; i < 10; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(heads[i], i * 2);
 				});
 				assert($.get(splited[i]) === i * 2 + 1);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_repeated.js b/benchmarking/benchmarks/reactivity/kairo/kairo_repeated.js
index 53b85acd3766..3bee06ca0e8f 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_repeated.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_repeated.js
@@ -25,13 +25,13 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			assert($.get(current) === size);
 			counter = 0;
 			for (let i = 0; i < 100; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(current) === i * size);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_triangle.js b/benchmarking/benchmarks/reactivity/kairo/kairo_triangle.js
index b9e2ad9fa4a3..11a419a52e7b 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_triangle.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_triangle.js
@@ -38,13 +38,13 @@ function setup() {
 		destroy,
 		run() {
 			const constant = count(width);
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			assert($.get(sum) === constant);
 			counter = 0;
 			for (let i = 0; i < 100; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 				assert($.get(sum) === constant - width + i * width);
diff --git a/benchmarking/benchmarks/reactivity/kairo/kairo_unstable.js b/benchmarking/benchmarks/reactivity/kairo/kairo_unstable.js
index 0e783732dc67..54eb732cb29d 100644
--- a/benchmarking/benchmarks/reactivity/kairo/kairo_unstable.js
+++ b/benchmarking/benchmarks/reactivity/kairo/kairo_unstable.js
@@ -25,13 +25,13 @@ function setup() {
 	return {
 		destroy,
 		run() {
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(head, 1);
 			});
 			assert($.get(current) === 40);
 			counter = 0;
 			for (let i = 0; i < 100; i++) {
-				$.flush_sync(() => {
+				$.flush(() => {
 					$.set(head, i);
 				});
 			}
diff --git a/benchmarking/benchmarks/reactivity/mol_bench.js b/benchmarking/benchmarks/reactivity/mol_bench.js
index c9f492f61967..536b078d74a4 100644
--- a/benchmarking/benchmarks/reactivity/mol_bench.js
+++ b/benchmarking/benchmarks/reactivity/mol_bench.js
@@ -51,11 +51,11 @@ function setup() {
 		 */
 		run(i) {
 			res.length = 0;
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(B, 1);
 				$.set(A, 1 + i * 2);
 			});
-			$.flush_sync(() => {
+			$.flush(() => {
 				$.set(A, 2 + i * 2);
 				$.set(B, 2);
 			});

From c55a698e29270acc3733526be1602b39ceca3c62 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 22:53:51 -0500
Subject: [PATCH 18/23] unused

---
 packages/svelte/src/internal/client/runtime.js | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index cba30376ab08..9d4448d845ca 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -650,7 +650,6 @@ function flush_queued_root_effects() {
 	is_flushing_effect = true;
 
 	try {
-		var length = queued_root_effects.length;
 		var flush_count = 0;
 
 		while (queued_root_effects.length > 0) {

From 763ed356882ed4f4a6866cef10e297166a331c96 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 23:00:33 -0500
Subject: [PATCH 19/23] simplify

---
 .../src/internal/client/reactivity/effects.js |  8 ------
 .../src/internal/client/reactivity/sources.js | 26 +++++++------------
 .../svelte/src/internal/client/runtime.js     | 12 +++------
 3 files changed, 13 insertions(+), 33 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 9d7b5e9de624..28589ce94df1 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -6,15 +6,12 @@ import {
 	update_effect,
 	get,
 	is_destroying_effect,
-	is_flushing_effect,
 	remove_reactions,
 	schedule_effect,
 	set_active_reaction,
 	set_is_destroying_effect,
-	set_is_flushing_effect,
 	set_signal_status,
 	untrack,
-	skip_reaction,
 	untracking
 } from '../runtime.js';
 import {
@@ -118,17 +115,12 @@ function create_effect(type, fn, sync, push = true) {
 	}
 
 	if (sync) {
-		var previously_flushing_effect = is_flushing_effect;
-
 		try {
-			set_is_flushing_effect(true);
 			update_effect(effect);
 			effect.f |= EFFECT_RAN;
 		} catch (e) {
 			destroy_effect(effect);
 			throw e;
-		} finally {
-			set_is_flushing_effect(previously_flushing_effect);
 		}
 	} else if (fn !== null) {
 		schedule_effect(effect);
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index ded0ca05846b..f6a3fd7e330a 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -14,8 +14,6 @@ import {
 	derived_sources,
 	set_derived_sources,
 	check_dirtiness,
-	set_is_flushing_effect,
-	is_flushing_effect,
 	untracking
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
@@ -202,22 +200,18 @@ export function internal_set(source, value) {
 
 		if (DEV && inspect_effects.size > 0) {
 			const inspects = Array.from(inspect_effects);
-			var previously_flushing_effect = is_flushing_effect;
-			set_is_flushing_effect(true);
-			try {
-				for (const effect of inspects) {
-					// Mark clean inspect-effects as maybe dirty and then check their dirtiness
-					// instead of just updating the effects - this way we avoid overfiring.
-					if ((effect.f & CLEAN) !== 0) {
-						set_signal_status(effect, MAYBE_DIRTY);
-					}
-					if (check_dirtiness(effect)) {
-						update_effect(effect);
-					}
+
+			for (const effect of inspects) {
+				// Mark clean inspect-effects as maybe dirty and then check their dirtiness
+				// instead of just updating the effects - this way we avoid overfiring.
+				if ((effect.f & CLEAN) !== 0) {
+					set_signal_status(effect, MAYBE_DIRTY);
+				}
+				if (check_dirtiness(effect)) {
+					update_effect(effect);
 				}
-			} finally {
-				set_is_flushing_effect(previously_flushing_effect);
 			}
+
 			inspect_effects.clear();
 		}
 	}
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 9d4448d845ca..589e6bb9dc31 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -53,11 +53,6 @@ let last_scheduled_effect = null;
 export let is_flushing_effect = false; // TODO do we still need this?
 export let is_destroying_effect = false;
 
-/** @param {boolean} value */
-export function set_is_flushing_effect(value) {
-	is_flushing_effect = value;
-}
-
 /** @param {boolean} value */
 export function set_is_destroying_effect(value) {
 	is_destroying_effect = value;
@@ -552,8 +547,10 @@ export function update_effect(effect) {
 
 	var previous_effect = active_effect;
 	var previous_component_context = component_context;
+	var previously_flushing_effect = is_flushing_effect;
 
 	active_effect = effect;
+	is_flushing_effect = true;
 
 	if (DEV) {
 		var previous_component_fn = dev_current_component_function;
@@ -595,6 +592,7 @@ export function update_effect(effect) {
 	} catch (error) {
 		handle_error(error, effect, previous_effect, previous_component_context || effect.ctx);
 	} finally {
+		is_flushing_effect = previously_flushing_effect;
 		active_effect = previous_effect;
 
 		if (DEV) {
@@ -646,9 +644,6 @@ function infinite_loop_guard() {
 }
 
 function flush_queued_root_effects() {
-	var previously_flushing_effect = is_flushing_effect;
-	is_flushing_effect = true;
-
 	try {
 		var flush_count = 0;
 
@@ -674,7 +669,6 @@ function flush_queued_root_effects() {
 			}
 		}
 	} finally {
-		is_flushing_effect = previously_flushing_effect;
 		is_flushing = false;
 
 		last_scheduled_effect = null;

From 9da49279495c43be0e0539d78f089898453db2d9 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 23:01:56 -0500
Subject: [PATCH 20/23] tidy up

---
 packages/svelte/src/internal/client/runtime.js | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 589e6bb9dc31..2f66d594c90f 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -50,7 +50,8 @@ let is_flushing = false;
 /** @type {Effect | null} */
 let last_scheduled_effect = null;
 
-export let is_flushing_effect = false; // TODO do we still need this?
+let is_updating_effect = false;
+
 export let is_destroying_effect = false;
 
 /** @param {boolean} value */
@@ -401,7 +402,7 @@ export function update_reaction(reaction) {
 	active_reaction = (flags & (BRANCH_EFFECT | ROOT_EFFECT)) === 0 ? reaction : null;
 	skip_reaction =
 		(flags & UNOWNED) !== 0 &&
-		(!is_flushing_effect || previous_reaction === null || previous_untracking);
+		(!is_updating_effect || previous_reaction === null || previous_untracking);
 
 	derived_sources = null;
 	set_component_context(reaction.ctx);
@@ -547,10 +548,10 @@ export function update_effect(effect) {
 
 	var previous_effect = active_effect;
 	var previous_component_context = component_context;
-	var previously_flushing_effect = is_flushing_effect;
+	var was_updating_effect = is_updating_effect;
 
 	active_effect = effect;
-	is_flushing_effect = true;
+	is_updating_effect = true;
 
 	if (DEV) {
 		var previous_component_fn = dev_current_component_function;
@@ -592,7 +593,7 @@ export function update_effect(effect) {
 	} catch (error) {
 		handle_error(error, effect, previous_effect, previous_component_context || effect.ctx);
 	} finally {
-		is_flushing_effect = previously_flushing_effect;
+		is_updating_effect = was_updating_effect;
 		active_effect = previous_effect;
 
 		if (DEV) {

From 026a1f2b00a7dd4a158d32fd80955ebf9674217a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 20 Feb 2025 23:08:41 -0500
Subject: [PATCH 21/23] tidy up

---
 packages/svelte/src/internal/client/runtime.js | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 2f66d594c90f..293d8c706d45 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -399,10 +399,9 @@ export function update_reaction(reaction) {
 	new_deps = /** @type {null | Value[]} */ (null);
 	skipped_deps = 0;
 	untracked_writes = null;
-	active_reaction = (flags & (BRANCH_EFFECT | ROOT_EFFECT)) === 0 ? reaction : null;
 	skip_reaction =
-		(flags & UNOWNED) !== 0 &&
-		(!is_updating_effect || previous_reaction === null || previous_untracking);
+		(flags & UNOWNED) !== 0 && (untracking || !is_updating_effect || active_reaction === null);
+	active_reaction = (flags & (BRANCH_EFFECT | ROOT_EFFECT)) === 0 ? reaction : null;
 
 	derived_sources = null;
 	set_component_context(reaction.ctx);

From a3aa4f3969c48e8328c0b52d0b5611e6678373f2 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 21 Feb 2025 08:00:57 -0500
Subject: [PATCH 22/23] present unnecessary microtasks, avoid flushing if no
 function provided

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

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 293d8c706d45..b5cd4b1536a3 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -823,13 +823,18 @@ function process_effects(effect) {
  * @returns {T extends void ? void : T}
  */
 export function flushSync(fn) {
-	flush_queued_root_effects();
+	var result;
 
-	var result = fn?.();
+	if (fn) {
+		is_flushing = true;
+		flush_queued_root_effects();
+		result = fn();
+	}
 
 	flush_tasks();
 
 	while (queued_root_effects.length > 0) {
+		is_flushing = true;
 		flush_queued_root_effects();
 		flush_tasks();
 	}

From 0b18d816be996e1e06c56d60ebcb1fecb7112037 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 21 Feb 2025 08:58:29 -0500
Subject: [PATCH 23/23] simplify

---
 packages/svelte/src/internal/client/runtime.js | 4 ++--
 packages/svelte/types/index.d.ts               | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index b5cd4b1536a3..cd3f794421b7 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -820,7 +820,7 @@ function process_effects(effect) {
  * Returns void if no callback is provided, otherwise returns the result of calling the callback.
  * @template [T=void]
  * @param {(() => T) | undefined} [fn]
- * @returns {T extends void ? void : T}
+ * @returns {T}
  */
 export function flushSync(fn) {
 	var result;
@@ -839,7 +839,7 @@ export function flushSync(fn) {
 		flush_tasks();
 	}
 
-	return /** @type {T extends void ? void : T} */ (result);
+	return /** @type {T} */ (result);
 }
 
 /**
diff --git a/packages/svelte/types/index.d.ts b/packages/svelte/types/index.d.ts
index 4bfb160395f1..4cb66aa87d30 100644
--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -421,7 +421,7 @@ declare module 'svelte' {
 	 * Synchronously flush any pending updates.
 	 * Returns void if no callback is provided, otherwise returns the result of calling the callback.
 	 * */
-	export function flushSync<T = void>(fn?: (() => T) | undefined): T extends void ? void : T;
+	export function flushSync<T = void>(fn?: (() => T) | undefined): T;
 	/**
 	 * Returns a promise that resolves once any pending state changes have been applied.
 	 * */