fix: don't notify SvelteMap key trackers when an existing key's value is set

.changeset/major-maps-agree.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't notify `SvelteMap` key trackers when an existing key's value is set

packages/svelte/src/reactivity/map.js

@@ -161,19 +161,23 @@ export class SvelteMap extends Map {
 		var sources = this.#sources;
 		var s = sources.get(key);
 		var prev_res = super.get(key);
+		var existed = super.has(key);
 		var res = super.set(key, value);
 		var version = this.#version;
 
 		if (s === undefined) {
-			s = this.#source(0);
+			// if the key existed but was never read individually, nothing tracks it and the key set is unchanged
+			if (!existed) {
+				s = this.#source(0);
 
-			if (DEV) {
-				tag(s, `SvelteMap get(${label(key)})`);
-			}
+				if (DEV) {
+					tag(s, `SvelteMap get(${label(key)})`);
+				}
 
-			sources.set(key, s);
-			set(this.#size, super.size);
-			increment(version);
+				sources.set(key, s);
+				set(this.#size, super.size);
+				increment(version);
+			}
 		} else if (prev_res !== value) {
 			increment(s);
 

packages/svelte/src/reactivity/map.test.ts

@@ -307,3 +307,33 @@ test('not invoking reactivity when value is not in the map after changes', () =>
 test('Map.instanceOf', () => {
 	assert.equal(new SvelteMap() instanceof Map, true);
 });
+
+test('map.keys() is not notified when an existing unread key is set to a new value', () => {
+	const map = new SvelteMap([
+		[1, 'a'],
+		[2, 'b']
+	]);
+
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push(Array.from(map.keys()));
+		});
+	});
+
+	flushSync(() => {
+		map.set(1, 'c');
+	});
+
+	flushSync(() => {
+		map.set(3, 'd');
+	});
+
+	assert.deepEqual(log, [
+		[1, 2],
+		[1, 2, 3]
+	]);
+
+	cleanup();
+});