fix

Author: dummdidumm

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

@@ -242,9 +242,13 @@ export function prop(props, key, flags, fallback) {
 
 	var get_fallback = () => {
 		fallback_used = true;
-		if (lazy && fallback_dirty) {
+		if (fallback_dirty) {
 			fallback_dirty = false;
-			fallback_value = untrack(/** @type {() => V} */ (fallback));
+			if (lazy) {
+				fallback_value = untrack(/** @type {() => V} */ (fallback));
+			} else {
+				fallback_value = /** @type {V} */ (fallback);
+			}
 		}
 
 		return fallback_value;
@@ -266,6 +270,7 @@ export function prop(props, key, flags, fallback) {
 			var value = /** @type {V} */ (props[key]);
 			if (value === undefined) return get_fallback();
 			fallback_dirty = true;
+			fallback_used = false;
 			return value;
 		};
 	} else {
@@ -355,7 +360,7 @@ export function prop(props, key, flags, fallback) {
 				set(inner_current_value, new_value);
 				// To ensure the fallback value is consistent when used with proxies, we
 				// update the local fallback_value, but only if the fallback is actively used
-				if (fallback_dirty && fallback_used && fallback_value !== undefined) {
+				if (fallback_used && fallback_value !== undefined) {
 					fallback_value = new_value;
 				}
 				get(current_value); // force a synchronisation immediately

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

@@ -5,11 +5,27 @@ export default test({
 	accessors: false,
 
 	test({ assert, target }) {
-		const [btn] = target.querySelectorAll('button');
+		const [btn1, btn2] = target.querySelectorAll('button');
 
-		btn.click();
+		btn1.click();
 		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>set color</button> <button>set options</button> bar bar`
+		);
 
-		assert.htmlEqual(target.innerHTML, `<button>change</button>\nbar\nbar`);
+		btn2.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>set color</button> <button>set options</button> baz bar`
+		);
+
+		btn1.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>set color</button> <button>set options</button> foo bar`
+		);
 	}
 });

packages/svelte/tests/runtime-runes/samples/props-default-value-rest/main.svelte

@@ -10,7 +10,14 @@
 	testProps = {
 		color: "blue"
 	};
-}}>change</button>
+}}>set color</button>
+
+<button onclick={() => {
+	testProps = {
+		color: "pink",
+		options: 'baz'
+	};
+}}>set options</button>
 
 <Inner {...testProps} />