From d175cc499d5f44db2d53ecd9316306dc03992e06 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 20 Mar 2026 12:06:10 +0100
Subject: [PATCH] fix

---
 .../internal/client/reactivity/deriveds.js    | 31 +++++--
 .../async-overlap-multiple-5/_config.js       | 87 +++++++++++++++++++
 .../async-overlap-multiple-5/main.svelte      | 23 +++++
 .../async-overlap-multiple-6/_config.js       | 76 ++++++++++++++++
 .../async-overlap-multiple-6/main.svelte      | 23 +++++
 .../async-overlap-multiple-7/_config.js       | 87 +++++++++++++++++++
 .../async-overlap-multiple-7/main.svelte      | 23 +++++
 7 files changed, 342 insertions(+), 8 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 07e7613e07..d86cef1237 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -44,6 +44,7 @@ import { batch_values, current_batch } from './batch.js';
 import { increment_pending, unset_context } from './async.js';
 import { deferred, includes, noop } from '../../shared/utils.js';
 import { set_signal_status, update_derived_status } from './status.js';
+import { queue_micro_task } from '../dom/task.js';
 
 /**
  * This allows us to track 'reactivity loss' that occurs when signals
@@ -127,7 +128,7 @@ export function async_derived(fn, label, location) {
 	/** @type {Map<Batch, ReturnType<typeof deferred<V>>>} */
 	var deferreds = new Map();
 
-	async_effect(() => {
+	var async_e = async_effect(() => {
 		if (DEV) {
 			reactivity_loss_tracker = {
 				effect: /** @type {Effect} */ (active_effect),
@@ -160,7 +161,8 @@ export function async_derived(fn, label, location) {
 		if (should_suspend) {
 			// we only increment the batch's pending state for updates, not creation, otherwise
 			// we will decrement to zero before the work that depends on this promise (e.g. a
-			// template effect) has initialized, causing the batch to resolve prematurely
+			// template effect) has initialized, causing the batch to resolve prematurely.
+			// Also see test async-overlap-multiple-6
 			if ((effect.f & REACTION_RAN) !== 0) {
 				var decrement_pending = increment_pending();
 			}
@@ -192,6 +194,21 @@ export function async_derived(fn, label, location) {
 
 			batch.register_async_derived(d.reject);
 			deferreds.set(batch, d);
+
+			// Check if a later batch started work earlier than an earlier one.
+			// This could happen when two batches write to the same async derived
+			// but the earlier one is only writing to it after going through another
+			// async derived, while the later one is writing to it immediately.
+			// In that case, to ensure the order is preserved and the later batch
+			// is invoked with the right values, we have to restart the later batch's async derived.
+			for (const b of deferreds.keys()) {
+				if (b === batch) break;
+				if (b.id > batch.id) {
+					set_signal_status(async_e, DIRTY);
+					b.schedule(async_e);
+					queue_micro_task(() => b.flush());
+				}
+			}
 		}
 
 		/**
@@ -212,12 +229,10 @@ export function async_derived(fn, label, location) {
 
 					// All prior async derived runs are now stale, but we have to
 					// wait for the corresponding batches to resolve before proceeding.
-					// We sort because batch order is important, not in which order the
-					// corresponding async work started.
-					for (const [b, d] of [...deferreds].sort(([a], [b]) => a.id - b.id)) {
-						if (b === batch) break;
-						waits.push(b.settled());
-						b.reject_async(d.reject);
+					for (const [other_batch, other_d] of deferreds) {
+						if (other_batch === batch) break;
+						waits.push(other_batch.settled());
+						other_batch.reject_async(other_d.reject);
 					}
 
 					if (waits.length > 0) {
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/_config.js b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/_config.js
new file mode 100644
index 0000000000..f07c86dc0c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/_config.js
@@ -0,0 +1,87 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 0 | d 2
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/main.svelte
new file mode 100644
index 0000000000..cf028718c2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-5/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/_config.js b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/_config.js
new file mode 100644
index 0000000000..27152889a2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/_config.js
@@ -0,0 +1,76 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		// Although the second batch is eventually connected to the first one, we can't see that
+		// at this point yet and so the second one flushes right away.
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 1 | d 1
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 1 | d 1
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/main.svelte
new file mode 100644
index 0000000000..cf028718c2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-6/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/_config.js b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/_config.js
new file mode 100644
index 0000000000..62ba0e3f46
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/_config.js
@@ -0,0 +1,87 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [a, c, shift, pop] = target.querySelectorAll('button');
+
+		a.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		c.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // schedules second step of first batch and schedules rerun of second batch
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		pop.click(); // second batch resolves but knows it needs to wait on first batch
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // obsolete second batch promise (already rejected)
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 0 | b 0 | c 0 | d 0
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+
+		shift.click(); // first batch resolves, with it second can now resolve as well
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			a 1 | b 2 | c 1 | d 3
+			<button>a++</button>
+			<button>c++</button>
+			<button>shift</button>
+			<button>pop</button>
+		`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/main.svelte
new file mode 100644
index 0000000000..cf028718c2
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-overlap-multiple-7/main.svelte
@@ -0,0 +1,23 @@
+<script>
+	let a = $state(0);
+	let b = $derived(await delay(a * 2));
+	let c = $state(0);
+	let d = $derived(await delay(b + c));
+
+	const deferred = [];
+
+	function delay(value) {
+		if (!value) return value; 
+		return new Promise((resolve) => deferred.push(() => resolve(value)));
+	}
+</script>
+
+a {a} | b {b} | c {c} | d {d}
+<button onclick={() => {a++;}}>
+	a++
+</button>
+<button onclick={() => {c++;}}>
+	c++
+</button>
+<button onclick={() => deferred.shift()?.()}>shift</button>
+<button onclick={() => deferred.pop()?.()}>pop</button>
\ No newline at end of file