From 4d611b8e40d41711eff78cee1bc7326752406318 Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Sun, 8 Jun 2025 23:58:02 +0100
Subject: [PATCH 1/3] Fix top-most derived in a chain of deriveds marked as
 MAYBE_DIRTY when executed from a snippet $.fallback

---
 .../svelte/src/internal/client/reactivity/deriveds.js  | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index e9cea0df3e64..33026e4039ca 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -183,8 +183,14 @@ export function update_derived(derived) {
 	// cleanup function, or it will cache a stale value
 	if (is_destroying_effect) return;
 
-	var status =
-		(skip_reaction || (derived.f & UNOWNED) !== 0) && derived.deps !== null ? MAYBE_DIRTY : CLEAN;
+	// only mark unowned deriveds as MAYBE_DIRTY if they have dependencies, otherwise they
+	// must be clean regardless of the value of the skip_reaction flag value set for the previous_reaction
+	// because not marking a regular derived as CLEAN will cause incosistent state when chaining
+	// multiple derivides in which the top-most derived is marked MAYBE_DIRTY and all the ones that depends
+	// on it are instead marked as CLEAN causing issues with properly updating the UI when the source state
+	// is updated because the MAYBE_DIRTY derived is skipped and as a consequence also
+	// the other deriveds (aka its reactions) are skipped as well.
+	var status = (derived.f & UNOWNED) !== 0 && derived.deps !== null ? MAYBE_DIRTY : CLEAN;
 
 	set_signal_status(derived, status);
 }

From f80082c58a73ba6f9ea52449355b7773ebc8e4fd Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Mon, 9 Jun 2025 00:10:09 +0100
Subject: [PATCH 2/3] Add test

---
 .../_config.js                                | 58 +++++++++++++++++++
 .../main.svelte                               | 33 +++++++++++
 2 files changed, 91 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/_config.js b/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/_config.js
new file mode 100644
index 000000000000..20df2425f6f6
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/_config.js
@@ -0,0 +1,58 @@
+import { test } from '../../test';
+import { flushSync, tick } from 'svelte';
+
+export default test({
+	async test({ assert, target }) {
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button id="step1">step1</button>
+			<button id="step2">step2</button>
+			<button id="step3">step3</button>
+			<p>0</p>
+		`
+		);
+
+		const step1 = /** @type {HTMLButtonElement | null} */ (target.querySelector('#step1'));
+		const step2 = /** @type {HTMLButtonElement | null} */ (target.querySelector('#step2'));
+		const step3 = /** @type {HTMLButtonElement | null} */ (target.querySelector('#step3'));
+
+		// Step 1: hide and reset data
+		step1?.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button id="step1">step1</button>
+			<button id="step2">step2</button>
+			<button id="step3">step3</button>
+		`
+		);
+
+		// Step 2: show again
+		step2?.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button id="step1">step1</button>
+			<button id="step2">step2</button>
+			<button id="step3">step3</button>
+			<p>0</p>
+		`
+		);
+
+		// Step 3: update override - this should show 2, not 0 (the bug)
+		step3?.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button id="step1">step1</button>
+			<button id="step2">step2</button>
+			<button id="step3">step3</button>
+			<p>2</p>
+		`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/main.svelte b/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/main.svelte
new file mode 100644
index 000000000000..3269ad28a050
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/devides-chain-with-snippet-fallback-trigger/main.svelte
@@ -0,0 +1,33 @@
+<script>
+	let show = $state(true);
+	let data = $state({ value: 0 });
+	let override = $state(null);
+
+	let derived1 = $derived(override ?? data.value);
+	let derived2 = $derived(derived1);
+
+	function step1() {
+		show = false;
+		data = { value: 0 };
+	}
+
+	function step2() {
+		show = true;
+	}
+
+	function step3() {
+		override = 1;
+		override = 2;
+	}
+</script>
+
+<button id="step1" onclick={step1}>step1</button>
+<button id="step2" onclick={step2}>step2</button>
+<button id="step3" onclick={step3}>step3</button>
+
+{#snippet dummy(value = 0)}{/snippet}
+
+{#if show}
+	<p>{derived2}</p>
+	{@render dummy(derived2 ? 0 : 0)}
+{/if}

From 77e211632a00421794d160864e8396b3441caa56 Mon Sep 17 00:00:00 2001
From: raythurnvoid <53383860+raythurnvoid@users.noreply.github.com>
Date: Mon, 9 Jun 2025 00:54:08 +0100
Subject: [PATCH 3/3] Add changeset

---
 .changeset/fresh-walls-wave.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/fresh-walls-wave.md

diff --git a/.changeset/fresh-walls-wave.md b/.changeset/fresh-walls-wave.md
new file mode 100644
index 000000000000..2e9c949c7c16
--- /dev/null
+++ b/.changeset/fresh-walls-wave.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Fix top-most derived in a chain of deriveds marked as MAYBE_DIRTY when executed from a snippet $.fallback