fix: remove source onchange from proxy on reassignment

packages/svelte/src/internal/client/reactivity/sources.js

@@ -27,7 +27,8 @@ import {
 	UNOWNED,
 	MAYBE_DIRTY,
 	BLOCK_EFFECT,
-	ROOT_EFFECT
+	ROOT_EFFECT,
+	PROXY_ONCHANGE_SYMBOL
 } from '../constants.js';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
@@ -239,6 +240,14 @@ export function internal_set(source, value) {
 	if (!source.equals(value)) {
 		var old_value = source.v;
 
+		if (old_value != null && source.o?.onchange) {
+			// @ts-ignore
+			const remove = old_value[PROXY_ONCHANGE_SYMBOL];
+			if (remove && typeof remove === 'function') {
+				remove(source.o?.onchange, true);
+			}
+		}
+
 		if (is_destroying_effect) {
 			old_values.set(source, value);
 		} else {

packages/svelte/tests/runtime-runes/samples/state-onchange-reassign-proxy/_config.js

@@ -0,0 +1,30 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const [btn, btn2, btn3, btn4] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn.click();
+		});
+		assert.deepEqual(logs, ['a']);
+
+		flushSync(() => {
+			btn2.click();
+		});
+		assert.deepEqual(logs, ['a', 'b', 'c']);
+		flushSync(() => {
+			btn3.click();
+		});
+		assert.deepEqual(logs, ['a', 'b', 'c', 'b', 'c']);
+		flushSync(() => {
+			btn4.click();
+		});
+		assert.deepEqual(logs, ['a', 'b', 'c', 'b', 'c', 'c']);
+		flushSync(() => {
+			btn2.click();
+		});
+		assert.deepEqual(logs, ['a', 'b', 'c', 'b', 'c', 'c', 'b']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-onchange-reassign-proxy/main.svelte

@@ -0,0 +1,30 @@
+<script>
+	let obj = { count: 0 };
+
+	let a = $state(obj, {
+	  onchange() {
+	    console.log('a');
+	  }
+	});
+	
+	let b = $state(obj, {
+	  onchange() {
+	    console.log('b');
+	  }
+	});
+	
+	let c = $state(b, {
+	  onchange() {
+	    console.log('c');
+	  }
+	});
+</script>
+
+<button onclick={()=> a.count++}>{a.count}</button>
+<button onclick={()=> b.count++}>{b.count}</button> 
+<button onclick={()=> c.count++}>{c.count}</button> 
+
+<!-- click this button, then click the b and c buttons. in theory
+     you should see either 'b changed' or 'c changed', but
+     instead clicking b causes both -->
+<button onclick={() => c = { count: c.count }}>unlink</button>