From 87336a9dbb638940dc0fb786b780010bc42ae011 Mon Sep 17 00:00:00 2001
From: Xia Chao <236466140+bun-unsafe@users.noreply.github.com>
Date: Sun, 6 Sep 2026 19:33:18 +0800
Subject: [PATCH] fix: avoid duplicate reaction when reconnecting a derived

When a derived is disconnected (its last reader is hidden), then its
dependency is changed and the reader is shown again, `reconnect`
unconditionally pushed the derived into the dependency's `reactions`
array. If it was already present, this left a duplicate entry that
`remove_reaction` only removes once, leaking the derived (and any
component it is owned by) after unmount.

Guard the push with a membership check so a derived is only ever
registered once per dependency.
---
 .changeset/lazy-rivers-connect.md              | 5 +++++
 packages/svelte/src/internal/client/runtime.js | 5 ++++-
 2 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/lazy-rivers-connect.md

diff --git a/.changeset/lazy-rivers-connect.md b/.changeset/lazy-rivers-connect.md
new file mode 100644
index 0000000000..c2486bdac7
--- /dev/null
+++ b/.changeset/lazy-rivers-connect.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid duplicate reaction when reconnecting a derived after its reader is hidden
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index d86c349445..6fe6feaae6 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -736,7 +736,10 @@ function reconnect(derived) {
 	if (derived.deps === null) return;
 
 	for (const dep of derived.deps) {
-		(dep.reactions ??= []).push(derived);
+		var reactions = (dep.reactions ??= []);
+		if (index_of.call(reactions, derived) === -1) {
+			reactions.push(derived);
+		}
 
 		if ((dep.f & DERIVED) !== 0 && (dep.f & CONNECTED) === 0) {
 			unfreeze_derived_effects(/** @type {Derived} */ (dep));