From 9f12fdd61f076983b5c0f6e399bfb95287a90fa6 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 13 Oct 2026 22:27:04 +0200
Subject: [PATCH] we need the "has pending batches" check to keep deriveds
 (maybe)dirty in more places after all

---
 .../src/internal/client/reactivity/batch.js   | 17 +++++++
 .../internal/client/reactivity/deriveds.js    |  8 ++++
 .../src/internal/client/reactivity/sources.js |  7 ++-
 .../svelte/src/internal/client/runtime.js     |  6 ++-
 .../_config.js                                | 30 +++++++++++++
 .../main.svelte                               | 44 +++++++++++++++++++
 .../_config.js                                | 31 +++++++++++++
 .../main.svelte                               | 43 ++++++++++++++++++
 8 files changed, 183 insertions(+), 3 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/main.svelte

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index f99cb64d66..f5ebecd3d6 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -132,6 +132,23 @@ export function own_value(signal) {
 		: signal;
 }
 
+/**
+ * Whether some batch's view of the world (may) differ from the real one: there's a batch that is going to be
+ * processed (again) in its own view, and another batch, or it's a fork. The batch that is being committed and
+ * the one that hasn't been processed yet don't count, their view is the real one. Deriveds that are computed
+ * from the real values (e.g. in an event handler) must not be marked clean then, as those batches wouldn't
+ * re-validate them in their view (see `update_derived`)
+ */
+export function has_pending_batches() {
+	for (var batch = first_batch; batch !== null; batch = batch.next) {
+		if (batch !== previous_batch && batch !== current_batch) {
+			return batch.is_fork || /** @type {Batch} */ (first_batch).next !== null;
+		}
+	}
+
+	return false;
+}
+
 /** @type {Effect | null} */
 let last_scheduled_effect = null;
 
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index df475312f3..826326d71b 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -48,6 +48,7 @@ import {
 	batch_values,
 	current_batch,
 	first_batch,
+	has_pending_batches,
 	own_value,
 	previous_batch,
 	read_batch_local_value
@@ -467,6 +468,13 @@ export function update_derived(derived) {
 		} else if (derived.v !== UNINITIALIZED) {
 			set_signal_status(derived, MAYBE_DIRTY);
 		}
+	} else if (has_pending_batches()) {
+		// Computed from the real values (e.g. in an event handler) while other batches are in flight.
+		// Don't mark it clean: those batches see other values for its dependencies, and unless they
+		// stashed a reaction that reads it, they wouldn't re-validate it and would use whatever value
+		// they hold for it — which can be stale, e.g. if the batch that dirtied it never evaluated it
+		// (see `async-derived-read-keeps-maybe-dirty`)
+		set_signal_status(derived, MAYBE_DIRTY);
 	} else {
 		update_derived_status(derived);
 	}
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 7485062d8b..e823fcb266 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -38,6 +38,7 @@ import {
 	schedule_effect,
 	legacy_updates,
 	current_batch,
+	has_pending_batches,
 	own_value
 } from './batch.js';
 import { proxy } from '../proxy.js';
@@ -208,7 +209,11 @@ export function internal_set(source, value, updated_during_traversal = null) {
 
 			// During time traveling we don't want to reset the status so that
 			// traversal of the graph in the other batches still happens
-			if (batch_values === null) {
+			if (
+				batch_values === null &&
+				// could also be "read outside of reactivity", e.g. in an event handler, while other batches are in flight
+				!has_pending_batches()
+			) {
 				update_derived_status(derived);
 			}
 		}
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index ca531ac737..3e4973861f 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -53,6 +53,7 @@ import {
 	current_batch,
 	first_batch,
 	flushSync,
+	has_pending_batches,
 	held_sources,
 	previous_batch,
 	read_batch_local_value,
@@ -188,8 +189,9 @@ export function is_dirty(reaction) {
 			// During time traveling we don't want to reset the status so that
 			// traversal of the graph in the other batches still happens. Effects
 			// can be reset because block/async effects execute right away and others
-			// are deferred and re-dirtied as needed.
-			(batch_values === null || (flags & DERIVED) === 0)
+			// are deferred and re-dirtied as needed. Deriveds also mustn't be reset
+			// outside of a batch's view while other batches are in flight (see `update_derived`)
+			((flags & DERIVED) === 0 || (batch_values === null && !has_pending_batches()))
 		) {
 			set_signal_status(reaction, CLEAN);
 		}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/_config.js
new file mode 100644
index 0000000000..dcec0fc11d
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/_config.js
@@ -0,0 +1,30 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Like `async-derived-read-keeps-maybe-dirty`, but the `<span>` also reads `s`, which makes it DIRTY
+// rather than MAYBE_DIRTY, so A doesn't evaluate `d` even when checking the `<span>` effect eagerly
+// (`is_dirty` returns right away). C's read of `d` must still not mark it as clean
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [s, u, d, , resolve_b] = target.querySelectorAll('button');
+
+		// batch A: `d` becomes 10, but is not evaluated yet (the `<span>` is deferred)
+		s.click();
+		await tick();
+
+		// batch B
+		u.click();
+		await tick();
+
+		// batch C writes `s` and reads `d` (20) while A and B are pending
+		d.click();
+		await tick();
+
+		// B sees A's change (but not C's), so `d` is 10 in its world
+		resolve_b.click();
+		await tick();
+		assert.equal(logs[0], 'probe 10');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/main.svelte
new file mode 100644
index 0000000000..2f6bc41e2f
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty-direct/main.svelte
@@ -0,0 +1,44 @@
+<script>
+	let s = $state(0);
+	let u = $state(0);
+	let w = $state(0);
+	let show = $state(true);
+	let d = $derived(s * 10);
+
+	const pending = new Map();
+
+	function delay(name, value) {
+		if (!value) return value;
+		return new Promise((resolve) => pending.set(name, () => resolve(value)));
+	}
+
+	function probe(value) {
+		console.log(`probe ${value}`);
+		return value;
+	}
+</script>
+
+<button onclick={() => s++}>s</button>
+<button onclick={() => u++}>u</button>
+<button
+	onclick={() => {
+		s++;
+		d;
+		w++;
+	}}>d</button
+>
+<button onclick={() => pending.get('a')?.()}>resolve a</button>
+<button onclick={() => pending.get('b')?.()}>resolve b</button>
+<button onclick={() => pending.get('c')?.()}>resolve c</button>
+
+<p>{await delay('a', s)}</p>
+<p>{await delay('c', w)}</p>
+<!-- inside a branch, so that B's traversal doesn't reach (and stash) the effect reading `d` -->
+{#if show}
+	<!-- reads `s` directly, so the effect is DIRTY, so even if we were to eagerly do is_dirty in batch A it wouldn't evaluate `d` -->
+	<span>{s}-{d}</span>
+{/if}
+
+{#if await delay('b', u)}
+	<i>{await probe(d)}</i>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/_config.js b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/_config.js
new file mode 100644
index 0000000000..e6a608cb8c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/_config.js
@@ -0,0 +1,31 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Like `async-derived-assign-keeps-maybe-dirty`, but C reads `d` in the event handler instead of
+// assigning to it. That records `d`'s stale value (A never evaluated it) as C's previous value, and
+// must not mark `d` as clean: B never stashed a reaction that reads `d`, so it wouldn't re-validate
+// it and would use that stale value
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [s, u, d, , resolve_b] = target.querySelectorAll('button');
+
+		// batch A: `d` becomes 10, but is not evaluated yet (the `<span>` is deferred)
+		s.click();
+		await tick();
+
+		// batch B
+		u.click();
+		await tick();
+
+		// batch C writes `s` and reads `d` (20) while A and B are pending
+		d.click();
+		await tick();
+
+		// B sees A's change (but not C's), so `d` is 10 in its world
+		resolve_b.click();
+		await tick();
+		assert.equal(logs[0], 'probe 10');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/main.svelte
new file mode 100644
index 0000000000..67f0978c54
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-derived-read-keeps-maybe-dirty/main.svelte
@@ -0,0 +1,43 @@
+<script>
+	let s = $state(0);
+	let u = $state(0);
+	let w = $state(0);
+	let show = $state(true);
+	let d = $derived(s * 10);
+
+	const pending = new Map();
+
+	function delay(name, value) {
+		if (!value) return value;
+		return new Promise((resolve) => pending.set(name, () => resolve(value)));
+	}
+
+	function probe(value) {
+		console.log(`probe ${value}`);
+		return value;
+	}
+</script>
+
+<button onclick={() => s++}>s</button>
+<button onclick={() => u++}>u</button>
+<button
+	onclick={() => {
+		s++;
+		d;
+		w++;
+	}}>d</button
+>
+<button onclick={() => pending.get('a')?.()}>resolve a</button>
+<button onclick={() => pending.get('b')?.()}>resolve b</button>
+<button onclick={() => pending.get('c')?.()}>resolve c</button>
+
+<p>{await delay('a', s)}</p>
+<p>{await delay('c', w)}</p>
+<!-- inside a branch, so that B's traversal doesn't reach (and stash) the effect reading `d` -->
+{#if show}
+	<span>{d}</span>
+{/if}
+
+{#if await delay('b', u)}
+	<i>{await probe(d)}</i>
+{/if}