From 9ffc13d7c3a9360591e1ad45209b10024e3a8333 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 8 May 2026 12:55:29 -0400
Subject: [PATCH] fix: remove `source.updated` stack traces after `flush`
 (#18196)
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Noticed that we're not actually doing anything with `source_stacks` — we
shadow the module-level declaration in `flush`, which means we just keep
appending to it and then clearing a different (and empty) set. As a
result, any source that ever gets an `updated` property never gets rid
of it. This probably causes a memory leak?

Anyway, this fixes it.

### Before submitting the PR, please make sure you do the following

- [ ] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`
---
 .changeset/red-crabs-ring.md                  |  5 ++++
 .../src/internal/client/reactivity/batch.js   | 26 ++++++++++++-------
 2 files changed, 21 insertions(+), 10 deletions(-)
 create mode 100644 .changeset/red-crabs-ring.md

diff --git a/.changeset/red-crabs-ring.md b/.changeset/red-crabs-ring.md
new file mode 100644
index 0000000000..82b53c5464
--- /dev/null
+++ b/.changeset/red-crabs-ring.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reset `source.updated` stack traces after `flush`
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 418e2a8f1a..dfa53788f3 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -85,7 +85,9 @@ export let collected_effects = null;
 export let legacy_updates = null;
 
 var flush_count = 0;
-var source_stacks = DEV ? new Set() : null;
+
+/** @type {Set<Value>} */
+var source_stacks = new Set();
 
 let uid = 1;
 
@@ -273,6 +275,14 @@ export class Batch {
 			infinite_loop_guard();
 		}
 
+		if (DEV) {
+			// track all the values that were updated during this flush,
+			// so that they can be reset afterwards
+			for (const value of this.current.keys()) {
+				source_stacks.add(value);
+			}
+		}
+
 		// we only reschedule previously-deferred effects if we expect
 		// to be able to run them after processing the batch
 		if (!this.#is_deferred()) {
@@ -377,12 +387,6 @@ export class Batch {
 		}
 
 		if (next_batch !== null) {
-			if (DEV) {
-				for (const source of this.current.keys()) {
-					/** @type {Set<Source>} */ (source_stacks).add(source);
-				}
-			}
-
 			next_batch.#process();
 		}
 	}
@@ -481,9 +485,11 @@ export class Batch {
 	}
 
 	flush() {
-		var source_stacks = DEV ? new Set() : null;
-
 		try {
+			if (DEV) {
+				source_stacks.clear();
+			}
+
 			is_processing = true;
 			current_batch = this;
 
@@ -501,7 +507,7 @@ export class Batch {
 			old_values.clear();
 
 			if (DEV) {
-				for (const source of /** @type {Set<Source>} */ (source_stacks)) {
+				for (const source of source_stacks) {
 					source.updated = null;
 				}
 			}