From bc51a4fae8079b1faadf070161615b093d925428 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 May 2026 16:24:24 -0400
Subject: [PATCH] fix: differentiate between stale and obsolete async deriveds

---
 .../src/internal/client/reactivity/async.js   |  4 ++--
 .../src/internal/client/reactivity/batch.js   |  5 ++---
 .../internal/client/reactivity/deriveds.js    | 21 ++++++++++---------
 3 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 6aea790c36..af9a4e6fb4 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -334,8 +334,8 @@ export function increment_pending() {
 	boundary.update_pending_count(1, batch);
 	batch.increment(blocking, effect);
 
-	return (skip = false) => {
+	return () => {
 		boundary.update_pending_count(-1, batch);
-		batch.decrement(blocking, effect, skip);
+		batch.decrement(blocking, effect);
 	};
 }
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 4239cda04b..f019b75aea 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -642,9 +642,8 @@ export class Batch {
 	/**
 	 * @param {boolean} blocking
 	 * @param {Effect} effect
-	 * @param {boolean} skip - whether to skip updates (because this is triggered by a stale reaction)
 	 */
-	decrement(blocking, effect, skip) {
+	decrement(blocking, effect) {
 		let pending_count = this.#pending.get(effect) ?? 0;
 
 		if (pending_count === 1) {
@@ -663,7 +662,7 @@ export class Batch {
 			}
 		}
 
-		if (this.#decrement_queued || skip) return;
+		if (this.#decrement_queued) return;
 		this.#decrement_queued = true;
 
 		queue_micro_task(() => {
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index eb934d96ff..9999c31152 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -100,6 +100,8 @@ export function derived(fn) {
 	return signal;
 }
 
+const OBSOLETE = {};
+
 /**
  * @template V
  * @param {() => V | Promise<V>} fn
@@ -180,13 +182,13 @@ export function async_derived(fn, label, location) {
 			}
 
 			if (/** @type {Boundary} */ (parent.b).is_rendered()) {
-				deferreds.get(batch)?.reject(STALE_REACTION);
+				deferreds.get(batch)?.reject(OBSOLETE);
 				deferreds.delete(batch); // delete to ensure correct order in Map iteration below
 			} else {
 				// While the boundary is still showing pending, a new run supersedes all older in-flight runs
 				// for this async expression. Cancel eagerly so resolution cannot commit stale values.
 				for (const d of deferreds.values()) {
-					d.reject(STALE_REACTION);
+					d.reject(OBSOLETE);
 				}
 				deferreds.clear();
 			}
@@ -203,15 +205,14 @@ export function async_derived(fn, label, location) {
 				reactivity_loss_tracker = null;
 			}
 
-			if (decrement_pending) {
-				// don't trigger an update if we're only here because
-				// the promise was superseded before it could resolve
-				var skip = error === STALE_REACTION;
-				decrement_pending(skip);
+			// if the promise was rejected by the user, via `getAbortSignal`, then
+			// wait for a subsequent resolution instead of flushing the batch
+			if (error === STALE_REACTION) {
+				return;
 			}
 
-			if (error === STALE_REACTION || (effect.f & DESTROYED) !== 0) {
-				return;
+			if (decrement_pending) {
+				decrement_pending();
 			}
 
 			batch.activate();
@@ -255,7 +256,7 @@ export function async_derived(fn, label, location) {
 
 	teardown(() => {
 		for (const d of deferreds.values()) {
-			d.reject(STALE_REACTION);
+			d.reject(OBSOLETE);
 		}
 	});