From b08aeee3cfaf923e670d4d23d0b62bba3948db8d Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 22 Sep 2026 10:51:23 +0200
Subject: [PATCH] fix more edge cases

---
 .../internal/client/dom/blocks/branches.js    | 11 +++++
 .../src/internal/client/reactivity/batch.js   | 16 +++++--
 .../_config.js                                | 38 +++++++++++++++
 .../main.svelte                               | 48 +++++++++++++++++++
 .../_config.js                                | 42 ++++++++++++++++
 .../main.svelte                               | 43 +++++++++++++++++
 .../async-merge-branch-reselection/_config.js | 31 ++++++++++++
 .../main.svelte                               | 24 ++++++++++
 .../Child.svelte                              | 10 ++++
 .../_config.js                                | 47 ++++++++++++++++++
 .../main.svelte                               | 31 ++++++++++++
 11 files changed, 338 insertions(+), 3 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/main.svelte

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index ed55b39974..ffc42fe5ad 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -190,6 +190,17 @@ export class BranchManager {
 		var defer = should_defer_append();
 		var first = false;
 
+		// Re-evaluating in the surviving batch supersedes selections made before a merge,
+		// even though those batches originally had newer IDs and still have commit callbacks.
+		for (const previous of this.#batches.keys()) {
+			for (let merged = previous.merged_into; merged !== null; merged = merged.merged_into) {
+				if (merged === batch) {
+					this.#batches.delete(previous);
+					break;
+				}
+			}
+		}
+
 		if (fn && !this.#onscreen.has(key) && !this.#offscreen.has(key)) {
 			first = true;
 
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 80176f3184..dfb8117b65 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -720,6 +720,11 @@ export class Batch {
 			}
 		}
 
+		this.#pending += batch.#pending;
+		for (const [effect, count] of batch.#blocking_pending) {
+			this.#blocking_pending.set(effect, (this.#blocking_pending.get(effect) ?? 0) + count);
+		}
+
 		for (const c of batch.#commit_callbacks) {
 			this.oncommit(() => c(batch));
 		}
@@ -848,7 +853,7 @@ export class Batch {
 			batch.current.clear();
 			batch.discard();
 		} else {
-			if (current) batch.current.set(source, current);
+			if (current && current[0] !== value) batch.current.set(source, current);
 			if (
 				!is_derived &&
 				(!current || current[0] !== value) &&
@@ -862,7 +867,7 @@ export class Batch {
 				batch.current.delete(source);
 				const b = batch;
 				queue_micro_task(() => {
-					if (b.mark(source, DIRTY)) {
+					if (b.linked && b.mark(source, DIRTY)) {
 						b.flush();
 					}
 				});
@@ -979,8 +984,13 @@ export class Batch {
 	 * @param {Set<Effect>} dirty_effects
 	 * @param {Set<Effect>} maybe_dirty_effects
 	 * @param {Set<Derived>} dirty_deriveds
+	 * @returns {void}
 	 */
 	transfer_effects(dirty_effects, maybe_dirty_effects, dirty_deriveds) {
+		if (this.merged_into) {
+			return this.merged_into.transfer_effects(dirty_effects, maybe_dirty_effects, dirty_deriveds);
+		}
+
 		for (const e of dirty_effects) {
 			this.#dirty_effects.add(e);
 		}
@@ -1594,7 +1604,7 @@ export function fork(fn) {
 					} else if (!is_derived) {
 						const b = next_batch;
 						queue_micro_task(() => {
-							if (b.mark(source, DIRTY)) {
+							if (b.linked && b.mark(source, DIRTY)) {
 								b.flush();
 							}
 						});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/_config.js b/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/_config.js
new file mode 100644
index 0000000000..45f2a13f23
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/_config.js
@@ -0,0 +1,38 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = `
+	<button>fork x = 1</button>
+	<button>update y and discard</button>
+	<button>commit y fork and discard</button>
+	<button>resolve requests</button>
+	<button>reset</button>
+`;
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [speculate, update, commit, resolve, reset] = target.querySelectorAll('button');
+
+		for (const action of [update, commit]) {
+			speculate.click();
+			await tick();
+			logs.length = 0;
+
+			// Discard before the queued fork revalidation runs. It must not restart
+			// the async effect and abort the real world's request, whether the write
+			// came from a normal update or another fork's commit.
+			action.click();
+			await tick();
+			resolve.click();
+			await tick();
+
+			assert.htmlEqual(target.innerHTML, `${buttons}<p>0/1</p>`);
+			assert.deepEqual(logs, ['0/1']);
+
+			reset.click();
+			await tick();
+		}
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/main.svelte
new file mode 100644
index 0000000000..197764c910
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-discard-revalidation/main.svelte
@@ -0,0 +1,48 @@
+<script>
+	import { fork, getAbortSignal } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let f;
+	const resolvers = [];
+
+	function load(x, y) {
+		console.log(`${x}/${y}`);
+		const signal = getAbortSignal();
+		if (y === 0) return `${x}/${y}`;
+
+		return new Promise((resolve, reject) => {
+			resolvers.push(() => resolve(`${x}/${y}`));
+			signal.addEventListener('abort', () => reject(signal.reason));
+		});
+	}
+
+	function speculate() {
+		f = fork(() => { x = 1; });
+	}
+
+	function update_and_discard(use_fork) {
+		if (use_fork) {
+			fork(() => { y = 1; }).commit();
+		} else {
+			y = 1;
+		}
+		f.discard();
+	}
+
+	function finish() {
+		for (const resolve of resolvers.splice(0)) resolve();
+	}
+
+	function reset() {
+		y = 0;
+	}
+</script>
+
+<button onclick={speculate}>fork x = 1</button>
+<button onclick={() => update_and_discard(false)}>update y and discard</button>
+<button onclick={() => update_and_discard(true)}>commit y fork and discard</button>
+<button onclick={finish}>resolve requests</button>
+<button onclick={reset}>reset</button>
+
+<p>{await load(x, y)}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/_config.js b/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/_config.js
new file mode 100644
index 0000000000..b2f791a40a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/_config.js
@@ -0,0 +1,42 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = `
+	<button>fork x = y = 1</button>
+	<button>set x = y = 1</button>
+	<button>set z = 1</button>
+	<button>commit fork</button>
+	<button>discard fork</button>
+`;
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [speculate, catch_up, update, commit, discard] = target.querySelectorAll('button');
+
+		speculate.click();
+		await tick();
+		catch_up.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>2</p>`);
+		logs.length = 0;
+
+		// All speculative writes are obsolete. Only the real world should react.
+		update.click();
+		await tick();
+		try {
+			assert.htmlEqual(target.innerHTML, `${buttons}<p>3</p>`);
+			assert.deepEqual(logs, ['1/1/1']);
+
+			// Committing an automatically discarded fork should be a no-op and not throw,
+			// as the user cannot really know that something got automatically discarded.
+			commit.click();
+			await tick();
+			assert.htmlEqual(target.innerHTML, `${buttons}<p>3</p>`);
+			assert.deepEqual(logs, ['1/1/1', 'committed']);
+		} finally {
+			discard.click();
+		}
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/main.svelte
new file mode 100644
index 0000000000..eeacc2c8d1
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-fork-obsolete-multiple-sources/main.svelte
@@ -0,0 +1,43 @@
+<script>
+	import { fork } from 'svelte';
+
+	let x = $state(0);
+	let y = $state(0);
+	let z = $state(0);
+	let f;
+
+	function load(x, y, z) {
+		console.log(`${x}/${y}/${z}`);
+		return x + y + z;
+	}
+
+	function speculate() {
+		f = fork(() => { x = 1; y = 1; });
+	}
+
+	function catch_up() {
+		x = 1;
+		y = 1;
+	}
+
+	function update() {
+		z = 1;
+	}
+
+	function discard() {
+		f.discard();
+	}
+
+	async function commit() {
+		await f.commit();
+		console.log('committed');
+	}
+</script>
+
+<button onclick={speculate}>fork x = y = 1</button>
+<button onclick={catch_up}>set x = y = 1</button>
+<button onclick={update}>set z = 1</button>
+<button onclick={commit}>commit fork</button>
+<button onclick={discard}>discard fork</button>
+
+<p>{await load(x, y, z)}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/_config.js b/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/_config.js
new file mode 100644
index 0000000000..c21996a615
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/_config.js
@@ -0,0 +1,31 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = `
+	<button>start A: x = 1</button>
+	<button>start B: show branch</button>
+	<button>resolve A</button>
+`;
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target }) {
+		await tick();
+		const [start_a, start_b, resolve] = target.querySelectorAll('button');
+
+		start_a.click();
+		await tick();
+		start_b.click();
+		await tick();
+
+		// B selected the empty branch before merging into A. A's async result
+		// now selects the content, which B's old commit callback must not remove.
+		// The two blocks register A's and B's callbacks in opposite orders.
+		resolve.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`${buttons}<p>1/1</p><strong>ready</strong><em>also ready</em>`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/main.svelte
new file mode 100644
index 0000000000..605d5d85ea
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-branch-reselection/main.svelte
@@ -0,0 +1,24 @@
+<script>
+	let x = $state(0);
+	let show = $state(false);
+	let done;
+
+	function delay(value) {
+		if (value === 0) return 0;
+		return new Promise(resolve => { done = () => resolve(value); });
+	}
+
+	let result = $derived(await delay(x));
+</script>
+
+<button onclick={() => x = 1}>start A: x = 1</button>
+<button onclick={() => show = true}>start B: show branch</button>
+<button onclick={() => done()}>resolve A</button>
+
+<p>{x}/{result}</p>
+{#if show && x > 0 && result > 0}
+	<strong>ready</strong>
+{/if}
+{#if x > 0 && show && result > 0}
+	<em>also ready</em>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/Child.svelte
new file mode 100644
index 0000000000..15ca4b6127
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/Child.svelte
@@ -0,0 +1,10 @@
+<script>
+	let { delay } = $props();
+	let result = $derived(await delay());
+
+	$effect(() => {
+		console.log(result);
+	});
+</script>
+
+<strong>{result}</strong>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/_config.js b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/_config.js
new file mode 100644
index 0000000000..9ed25aba6a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/_config.js
@@ -0,0 +1,47 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons = `
+	<button>start A: x = 1</button>
+	<button>start B: show boundary</button>
+	<button>resolve A</button>
+	<button>resolve boundary</button>
+	<button>reset</button>
+`;
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [start_a, start_b, resolve_a, resolve_b, reset] = target.querySelectorAll('button');
+
+		for (const boundary_first of [false, true]) {
+			logs.length = 0;
+			start_a.click();
+			await tick();
+			start_b.click();
+			await tick();
+
+			// B's pending boundary must transfer effects to A regardless of whether
+			// it resolves before or after A renders.
+			if (boundary_first) {
+				resolve_b.click();
+				await tick();
+				assert.htmlEqual(target.innerHTML, `${buttons}<p>0</p>`);
+				assert.deepEqual(logs, []);
+				resolve_a.click();
+			} else {
+				resolve_a.click();
+				await tick();
+				assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><span>pending</span>`);
+				resolve_b.click();
+			}
+			await tick();
+			assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><strong>ready</strong>`);
+			assert.deepEqual(logs, ['ready']);
+
+			reset.click();
+			await tick();
+		}
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/main.svelte
new file mode 100644
index 0000000000..b38500ee63
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-merge-pending-boundary-effects/main.svelte
@@ -0,0 +1,31 @@
+<script>
+	import Child from './Child.svelte';
+
+	let x = $state(0);
+	let show = $state(false);
+	let resolve_a;
+	let resolve_b;
+
+	function delay_a(value) {
+		if (value === 0) return 0;
+		return new Promise(resolve => { resolve_a = () => resolve(value); });
+	}
+
+	function delay_b() {
+		return new Promise(resolve => { resolve_b = () => resolve('ready'); });
+	}
+</script>
+
+<button onclick={() => x = 1}>start A: x = 1</button>
+<button onclick={() => show = true}>start B: show boundary</button>
+<button onclick={() => resolve_a()}>resolve A</button>
+<button onclick={() => resolve_b()}>resolve boundary</button>
+<button onclick={() => { show = false; x = 0; }}>reset</button>
+
+<p>{await delay_a(x)}</p>
+{#if show && x > 0}
+	<svelte:boundary>
+		{#snippet pending()}<span>pending</span>{/snippet}
+		<Child delay={delay_b} />
+	</svelte:boundary>
+{/if}