From 2071160ce89dab1834c48042b739c725ff634597 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sat, 31 May 2025 21:04:15 -0400
Subject: [PATCH] keep order

---
 .../src/internal/client/reactivity/batch.js   | 95 ++++++++++++++-----
 .../_config.js                                | 40 ++++++++
 .../main.svelte                               | 17 ++++
 3 files changed, 126 insertions(+), 26 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 7f5cdea1a1..8653bafb5c 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -86,18 +86,61 @@ export class Batch {
 		}
 
 		if (this.async_effects.length === 0 && this.settled()) {
-			var render_effects = this.render_effects;
-			var effects = this.effects;
+			var merged = false;
+
+			// if there are older batches with overlapping
+			// state, we can't commit this batch. instead,
+			// we merge it into the older batches
+			for (const batch of batches) {
+				if (batch === this) break;
+
+				for (const [source] of batch.#current) {
+					if (this.#current.has(source)) {
+						merged = true;
+
+						for (const [source, value] of this.#current) {
+							batch.#current.set(source, value);
+							// TODO what about batch.#previous?
+						}
+
+						for (const e of this.render_effects) {
+							set_signal_status(e, CLEAN);
+							// TODO use sets instead of arrays
+							if (!batch.render_effects.includes(e)) {
+								batch.render_effects.push(e);
+							}
+						}
+
+						for (const e of this.effects) {
+							set_signal_status(e, CLEAN);
+							// TODO use sets instead of arrays
+							if (!batch.effects.includes(e)) {
+								batch.effects.push(e);
+							}
+						}
+
+						this.remove();
+						break;
+					}
+				}
+			}
 
-			this.render_effects = [];
-			this.effects = [];
+			if (merged) {
+				this.remove();
+			} else {
+				var render_effects = this.render_effects;
+				var effects = this.effects;
 
-			this.commit();
+				this.render_effects = [];
+				this.effects = [];
+
+				this.commit();
 
-			flush_queued_effects(render_effects);
-			flush_queued_effects(effects);
+				flush_queued_effects(render_effects);
+				flush_queued_effects(effects);
 
-			this.deferred?.resolve();
+				this.deferred?.resolve();
+			}
 		} else {
 			for (const e of this.render_effects) set_signal_status(e, CLEAN);
 			for (const e of this.effects) set_signal_status(e, CLEAN);
@@ -133,24 +176,24 @@ export class Batch {
 	remove() {
 		batches.delete(this);
 
-		for (var batch of batches) {
-			/** @type {Source} */
-			var source;
-
-			if (batch.#id < this.#id) {
-				// other batch is older than this
-				for (source of this.#previous.keys()) {
-					batch.#previous.delete(source);
-				}
-			} else {
-				// other batch is newer than this
-				for (source of batch.#previous.keys()) {
-					if (this.#previous.has(source)) {
-						batch.#previous.set(source, source.v);
-					}
-				}
-			}
-		}
+		// for (var batch of batches) {
+		// 	/** @type {Source} */
+		// 	var source;
+
+		// 	if (batch.#id < this.#id) {
+		// 		// other batch is older than this
+		// 		for (source of this.#previous.keys()) {
+		// 			batch.#previous.delete(source);
+		// 		}
+		// 	} else {
+		// 		// other batch is newer than this
+		// 		for (source of batch.#previous.keys()) {
+		// 			if (this.#previous.has(source)) {
+		// 				batch.#previous.set(source, source.v);
+		// 			}
+		// 		}
+		// 	}
+		// }
 	}
 
 	restore() {
diff --git a/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/_config.js b/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/_config.js
new file mode 100644
index 0000000000..e4d6979acf
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/_config.js
@@ -0,0 +1,40 @@
+import { flushSync, settled, tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>both</button><button>a</button><button>b</button><p>loading...</p>`,
+
+	async test({ assert, target }) {
+		const [both, a, b] = target.querySelectorAll('button');
+
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>both</button><button>a</button><button>b</button>
+				<p>1 * 2 = 2</p>
+				<p>2 * 2 = 4</p>
+			`
+		);
+
+		flushSync(() => both.click());
+		flushSync(() => b.click());
+
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>both</button><button>a</button><button>b</button>
+				<p>2 * 2 = 4</p>
+				<p>4 * 2 = 8</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/main.svelte
new file mode 100644
index 0000000000..432eed976c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-linear-order-different-deriveds/main.svelte
@@ -0,0 +1,17 @@
+<script>
+	let a = $state(1);
+	let b = $state(2);
+</script>
+
+<button onclick={() => {a += 1; b += 1;}}>both</button>
+<button onclick={() => {a += 1;}}>a</button>
+<button onclick={() => {b += 1;}}>b</button>
+
+<svelte:boundary>
+	<p>{a} * 2 = {await (a * 2)}</p>
+	<p>{b} * 2 = {b * 2}</p>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>