restore first_batch?.next check when assigning to a derived in internal_set

Marking the derived clean while other batches are in flight hides changes they haven't seen yet.

packages/svelte/src/internal/client/reactivity/sources.js

@@ -37,7 +37,8 @@ import {
 	eager_block_effects,
 	schedule_effect,
 	legacy_updates,
-	current_batch
+	current_batch,
+	first_batch
 } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
@@ -206,7 +207,11 @@ 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
+				!first_batch?.next
+			) {
 				update_derived_status(derived);
 			}
 		}

packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/_config.js

@@ -0,0 +1,39 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+const buttons =
+	'<button>s</button><button>u</button><button>d</button><button>resolve a</button><button>resolve b</button><button>resolve c</button>';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, target, logs }) {
+		await tick();
+		const [s, u, d, resolve_a, resolve_b, resolve_c] = 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 overrides `d` while A and B are pending. This must not mark `d` as
+		// clean, because the other batches haven't seen its new value (10) yet
+		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');
+
+		resolve_a.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>0</p><span>10</span><i>10</i>`);
+
+		resolve_c.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `${buttons}<p>1</p><p>1</p><span>99</span><i>99</i>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-assign-keeps-maybe-dirty/main.svelte

@@ -0,0 +1,42 @@
+<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={() => {
+		d = 99;
+		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}