chore: avoid microtasks when flushing sync

.changeset/polite-melons-tickle.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: avoid microtasks when flushing sync

benchmarking/benchmarks/reactivity/index.js

@@ -24,32 +24,32 @@ import {
 // Not all tests are the same, and many parts have been tweaked to capture different data.
 
 export const reactivity_benchmarks = [
-	sbench_create_signals,
+	// sbench_create_signals,
-	sbench_create_0to1,
+	// sbench_create_0to1,
-	sbench_create_1to1,
+	// sbench_create_1to1,
-	sbench_create_2to1,
+	// sbench_create_2to1,
-	sbench_create_4to1,
+	// sbench_create_4to1,
-	sbench_create_1000to1,
+	// sbench_create_1000to1,
-	sbench_create_1to2,
+	// sbench_create_1to2,
-	sbench_create_1to4,
+	// sbench_create_1to4,
-	sbench_create_1to8,
+	// sbench_create_1to8,
-	sbench_create_1to1000,
+	// sbench_create_1to1000,
-	kairo_avoidable_owned,
+	// kairo_avoidable_owned,
-	kairo_avoidable_unowned,
+	// kairo_avoidable_unowned,
-	kairo_broad_owned,
+	// kairo_broad_owned,
-	kairo_broad_unowned,
+	// kairo_broad_unowned,
-	kairo_deep_owned,
+	// kairo_deep_owned,
-	kairo_deep_unowned,
+	// kairo_deep_unowned,
 	kairo_diamond_owned,
 	kairo_diamond_unowned,
-	kairo_triangle_owned,
+	// kairo_triangle_owned,
-	kairo_triangle_unowned,
+	// kairo_triangle_unowned,
-	kairo_mux_owned,
+	// kairo_mux_owned,
-	kairo_mux_unowned,
+	// kairo_mux_unowned,
-	kairo_repeated_owned,
+	// kairo_repeated_owned,
-	kairo_repeated_unowned,
+	// kairo_repeated_unowned,
-	kairo_unstable_owned,
+	// kairo_unstable_owned,
-	kairo_unstable_unowned,
+	// kairo_unstable_unowned,
-	mol_bench_owned,
+	// mol_bench_owned,
-	mol_bench_unowned
+	// mol_bench_unowned
 ];

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

@@ -47,6 +47,7 @@ const handled_errors = new WeakSet();
 let is_throwing_error = false;
 
 let is_flushing = false;
+let is_flushing_sync = false;
 
 /** @type {Effect | null} */
 let last_scheduled_effect = null;
@@ -734,7 +735,9 @@ function flush_queued_effects(effects) {
 export function schedule_effect(signal) {
 	if (!is_flushing) {
 		is_flushing = true;
-		queueMicrotask(flush_queued_root_effects);
+		if (!is_flushing_sync) {
+			queueMicrotask(flush_queued_root_effects);
+		}
 	}
 
 	var effect = (last_scheduled_effect = signal);
@@ -818,23 +821,30 @@ function process_effects(root) {
  * @returns {T}
  */
 export function flushSync(fn) {
-	var result;
+	var previously_flushing_sync = is_flushing_sync;
-
-	if (fn) {
-		is_flushing = true;
-		flush_queued_root_effects();
-		result = fn();
-	}
 
-	flush_tasks();
+	try {
+		var result;
+		is_flushing_sync = true;
 
-	while (queued_root_effects.length > 0) {
+		if (fn) {
-		is_flushing = true;
+			is_flushing = true;
-		flush_queued_root_effects();
+			flush_queued_root_effects();
+			result = fn();
+		}
+	
 		flush_tasks();
+	
+		while (queued_root_effects.length > 0) {
+			is_flushing = true;
+			flush_queued_root_effects();
+			flush_tasks();
+		}
+	
+		return /** @type {T} */ (result);
+	} finally {
+		is_flushing_sync = previously_flushing_sync;
 	}
-
-	return /** @type {T} */ (result);
 }
 
 /**