fix: keep fallback value after spread update not setting that prop (#9717)

fixes #9716

.changeset/ninety-dingos-walk.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep fallback value after spread update not setting that prop

packages/svelte/src/internal/client/runtime.js

@@ -1430,6 +1430,7 @@ export function prop_source(props_obj, key, default_value, call_default_value) {
 		.i;
 	let ignore_next1 = false;
 	let ignore_next2 = false;
+	let did_update_to_defined = !should_set_default_value;
 
 	let mount = true;
 	sync_effect(() => {
@@ -1445,8 +1446,13 @@ export function prop_source(props_obj, key, default_value, call_default_value) {
 			return;
 		}
 
-		if (not_equal(immutable, propagating_value, source_signal.v)) {
+		if (
+			// Ensure that updates from undefined to undefined are ignored
+			(did_update_to_defined || propagating_value !== undefined) &&
+			not_equal(immutable, propagating_value, source_signal.v)
+		) {
 			ignore_next2 = true;
+			did_update_to_defined = true;
 			// TODO figure out why we need it this way and the explain in a comment;
 			// some tests fail is we just do set_signal_value(source_signal, propagating_value)
 			untrack(() => set_signal_value(source_signal, propagating_value));
@@ -1469,6 +1475,7 @@ export function prop_source(props_obj, key, default_value, call_default_value) {
 
 			if (not_equal(immutable, propagating_value, possible_signal.v)) {
 				ignore_next1 = true;
+				did_update_to_defined = true;
 				untrack(() => update_bound_prop(propagating_value));
 			}
 		});

packages/svelte/tests/runtime-runes/samples/props-spread-fallback/Component.svelte

@@ -0,0 +1,5 @@
+<script>
+	const { propA, propB = "fallback" } = $props();
+</script>
+
+<p>{propA} {propB}</p>

packages/svelte/tests/runtime-runes/samples/props-spread-fallback/_config.js

@@ -0,0 +1,55 @@
+import { test } from '../../test';
+
+// Tests that fallback values are kept as long as the prop is not defined in the case of a spread
+export default test({
+	accessors: false, // so that propA actually becomes $.prop and not $.prop_source
+	html: `
+		<button>change propA</button>
+		<button>change propB</button>
+		<p>true fallback</p>
+	`,
+
+	async test({ assert, target }) {
+		const [propA, propB] = target.querySelectorAll('button');
+
+		await propA.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>change propA</button>
+				<button>change propB</button>
+				<p>false fallback</p>
+			`
+		);
+
+		await propB.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>change propA</button>
+				<button>change propB</button>
+				<p>false defined</p>
+			`
+		);
+
+		await propA.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>change propA</button>
+				<button>change propB</button>
+				<p>true defined</p>
+			`
+		);
+
+		await propB.click();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>change propA</button>
+				<button>change propB</button>
+				<p>true</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/props-spread-fallback/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	import Component from './Component.svelte';
+
+	let props = $state({
+		propA: true,
+		propB: undefined
+	});
+</script>
+
+<button on:click={() => {props.propA = !props.propA}}>change propA</button>
+<button on:click={() => {props.propB = props.propB ? undefined : 'defined'}}>change propB</button>
+<Component {...props}/>