From 1032b313aba1b79e0a942289dceb4eafc62f9096 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 4 Nov 2025 16:53:10 +0100
Subject: [PATCH] fix WAS_MARKED logic

---
 .../src/internal/client/reactivity/sources.js |  5 +++-
 .../svelte/src/internal/client/runtime.js     | 29 +++++++++++++++----
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index f3568325cb..0b5ad33bfc 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -335,7 +335,10 @@ function mark_reactions(signal, status) {
 
 		if ((flags & DERIVED) !== 0) {
 			if ((flags & WAS_MARKED) === 0) {
-				reaction.f |= WAS_MARKED;
+				// Only connected deriveds can be reliably unmarked right away
+				if (flags & CONNECTED) {
+					reaction.f |= WAS_MARKED;
+				}
 				mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
 			}
 		} else if (not_dirty) {
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index ed15213545..3a31c175f1 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -154,13 +154,13 @@ export function is_dirty(reaction) {
 		return true;
 	}
 
+	if (flags & DERIVED) {
+		reaction.f &= ~WAS_MARKED;
+	}
+
 	if ((flags & MAYBE_DIRTY) !== 0) {
 		var dependencies = reaction.deps;
 
-		if (flags & DERIVED) {
-			reaction.f &= ~WAS_MARKED;
-		}
-
 		if (dependencies !== null) {
 			var i;
 			var dependency;
@@ -365,9 +365,10 @@ function remove_reaction(signal, dependency) {
 	) {
 		set_signal_status(dependency, MAYBE_DIRTY);
 		// If we are working with a derived that is owned by an effect, then mark it as being
-		// disconnected.
+		// disconnected and remove the mark flag, as it cannot be reliably removed otherwise
 		if ((dependency.f & CONNECTED) !== 0) {
 			dependency.f ^= CONNECTED;
+			dependency.f &= ~WAS_MARKED;
 		}
 		// Disconnect any reactions owned by this reaction
 		destroy_derived_effects(/** @type {Derived} **/ (dependency));
@@ -613,6 +614,10 @@ export function get(signal) {
 		var should_reconnect = is_updating_effect && effect_tracking() && (derived.f & CONNECTED) === 0;
 
 		if (batch_values?.has(derived)) {
+			// This happens as part of is_dirty normally, but we return early
+			// here so we need to do it separately
+			remove_marked_flag(derived);
+
 			if (should_reconnect) {
 				reconnect(derived);
 			}
@@ -657,6 +662,20 @@ function reconnect(derived) {
 	}
 }
 
+/**
+ * Removes the WAS_MARKED flag from the derived and its dependencies
+ * @param {Derived} derived
+ */
+function remove_marked_flag(derived) {
+	if ((derived.f & WAS_MARKED) === 0) return;
+	derived.f ^= WAS_MARKED;
+
+	// Only deriveds with dependencies can be marked
+	for (const dep of /** @type {Value[]} */ (derived.deps)) {
+		remove_marked_flag(/** @type {Derived} */ (dep));
+	}
+}
+
 /** @param {Derived} derived */
 function depends_on_old_values(derived) {
 	if (derived.v === UNINITIALIZED) return true; // we don't know, so assume the worst