From 18ecf0138291f4e09a44cfc9a6facd2e683b04d9 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 16 Oct 2025 17:28:35 -0400
Subject: [PATCH] WIP

---
 .../src/internal/client/reactivity/async.js   |  6 ++++--
 .../src/internal/client/reactivity/batch.js   | 21 ++++++++++++++++---
 .../internal/client/reactivity/deriveds.js    |  6 ++++--
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index b1e8177949..a223d1b5be 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -203,8 +203,10 @@ export async function async_body(fn) {
 	var boundary = get_boundary();
 	var batch = /** @type {Batch} */ (current_batch);
 
+	var blocking = !boundary.is_pending();
+
 	boundary.update_pending_count(1);
-	batch.increment();
+	batch.increment(blocking);
 
 	var active = /** @type {Effect} */ (active_effect);
 
@@ -237,7 +239,7 @@ export async function async_body(fn) {
 		}
 
 		boundary.update_pending_count(-1);
-		batch.decrement();
+		batch.decrement(blocking);
 
 		unset_context();
 	}
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index c429b23c98..413cf63284 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -101,6 +101,11 @@ export class Batch {
 	 */
 	#pending = 0;
 
+	/**
+	 * The number of async effects that are currently in flight, _not_ inside a pending boundary
+	 */
+	#blocking_pending = 0;
+
 	/**
 	 * A deferred that resolves when the batch is committed, used with `settled()`
 	 * TODO replace with Promise.withResolvers once supported widely enough
@@ -257,7 +262,7 @@ export class Batch {
 			}
 		}
 
-		if (should_defer) {
+		if (this.#blocking_pending > 0) {
 			this.#defer_effects(target.effects);
 			this.#defer_effects(target.render_effects);
 			this.#defer_effects(target.block_effects);
@@ -407,12 +412,22 @@ export class Batch {
 		this.#deferred?.resolve();
 	}
 
-	increment() {
+	/**
+	 *
+	 * @param {boolean} blocking
+	 */
+	increment(blocking) {
 		this.#pending += 1;
+		if (blocking) this.#blocking_pending += 1;
 	}
 
-	decrement() {
+	/**
+	 *
+	 * @param {boolean} blocking
+	 */
+	decrement(blocking) {
 		this.#pending -= 1;
+		if (blocking) this.#blocking_pending -= 1;
 
 		for (const e of this.#dirty_effects) {
 			set_signal_status(e, DIRTY);
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index e9d5edca05..1989220abe 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -138,8 +138,10 @@ export function async_derived(fn, location) {
 		var batch = /** @type {Batch} */ (current_batch);
 
 		if (should_suspend) {
+			var blocking = !boundary.is_pending();
+
 			boundary.update_pending_count(1);
-			batch.increment();
+			batch.increment(blocking);
 
 			deferreds.get(batch)?.reject(STALE_REACTION);
 			deferreds.delete(batch); // delete to ensure correct order in Map iteration below
@@ -190,7 +192,7 @@ export function async_derived(fn, location) {
 
 			if (should_suspend) {
 				boundary.update_pending_count(-1);
-				batch.decrement();
+				batch.decrement(blocking);
 			}
 		};