From 12160e0c33c08d4a59361b13de0bbe0ea8a40866 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 15 Jul 2025 16:44:20 -0400
Subject: [PATCH] WIP sync effect pending updates

---
 .../src/internal/client/dom/blocks/boundary.js    | 15 +++++++++------
 .../src/internal/client/reactivity/batch.js       |  9 +++++++++
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 5ed0515210..85f52d956b 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -22,10 +22,11 @@ import { get_next_sibling } from '../operations.js';
 import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import { DEV } from 'esm-env';
-import { Batch } from '../../reactivity/batch.js';
+import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
+import { raf } from '../../timing.js';
 
 /**
  * @typedef {{
@@ -92,6 +93,12 @@ export class Boundary {
 	 */
 	#effect_pending = null;
 
+	#effect_pending_update = () => {
+		if (this.#effect_pending) {
+			internal_set(this.#effect_pending, this.#pending_count);
+		}
+	};
+
 	#effect_pending_subscriber = createSubscriber(() => {
 		this.#effect_pending = source(this.#pending_count);
 
@@ -238,11 +245,7 @@ export class Boundary {
 			this.parent.#update_pending_count(d);
 		}
 
-		queueMicrotask(() => {
-			if (this.#effect_pending) {
-				internal_set(this.#effect_pending, this.#pending_count);
-			}
-		});
+		effect_pending_updates.add(this.#effect_pending_update);
 	}
 
 	get_effect_pending() {
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 1126946ce9..2189535adb 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -49,6 +49,9 @@ export let batch_deriveds = null;
 /** @type {Effect[]} Stack of effects, dev only */
 export let dev_effect_stack = [];
 
+/** @type {Set<() => void>} */
+export let effect_pending_updates = new Set();
+
 /** @type {Effect[]} */
 let queued_root_effects = [];
 
@@ -320,6 +323,12 @@ export class Batch {
 		}
 
 		current_batch = null;
+
+		for (const update of effect_pending_updates) {
+			effect_pending_updates.delete(update);
+			update();
+			break;
+		}
 	}
 
 	flush_effects() {