fix: keep default values of props a proxy after reassignment

.changeset/empty-horses-tell.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: keep default values of props a proxy after reassignment

packages/svelte/src/compiler/phases/3-transform/client/utils.js

@@ -292,7 +292,17 @@ export function serialize_set_binding(node, context, fallback, options) {
 
 	const serialize = () => {
 		if (left === node.left) {
-			if (binding.kind === 'prop' || binding.kind === 'bindable_prop') {
+			const initial_bindable_proxy =
+				binding.initial &&
+				binding.initial.type !== 'ClassDeclaration' &&
+				binding.initial.type !== 'FunctionDeclaration' &&
+				binding.initial.type !== 'ImportDeclaration' &&
+				binding.initial.type !== 'EachBlock' &&
+				should_proxy_or_freeze(binding.initial, context.state.scope);
+			if (
+				binding.kind === 'prop' ||
+				(binding.kind === 'bindable_prop' && !initial_bindable_proxy)
+			) {
 				return b.call(left, value);
 			} else if (is_store) {
 				return b.call('$.store_set', serialize_get_binding(b.id(left_name), state), value);
@@ -318,6 +328,15 @@ export function serialize_set_binding(node, context, fallback, options) {
 							? b.call('$.freeze', value)
 							: value
 					);
+				} else if (initial_bindable_proxy) {
+					call = b.call(
+						left,
+						context.state.analysis.runes &&
+							!options?.skip_proxy_and_freeze &&
+							should_proxy_or_freeze(value, context.state.scope)
+							? serialize_proxy_reassignment(value, left_name, state)
+							: value
+					);
 				} else {
 					call = b.call('$.set', b.id(left_name), value);
 				}

packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js

@@ -33,5 +33,17 @@ export default test({
 				<button>reassign: 2</button>
 			`
 		);
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>mutate: 3</button>
+				<button>reassign: 3</button>
+			`
+		);
 	}
 });