fix: ensure props passed to components via mount are updateable

Fixes #14161

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

@@ -20,7 +20,7 @@ import {
 } from '../runtime.js';
 import { safe_equals } from './equality.js';
 import * as e from '../errors.js';
-import { BRANCH_EFFECT, LEGACY_DERIVED_PROP, ROOT_EFFECT } from '../constants.js';
+import { BRANCH_EFFECT, LEGACY_DERIVED_PROP, ROOT_EFFECT, STATE_SYMBOL } from '../constants.js';
 import { proxy } from '../proxy.js';
 import { capture_store_binding } from './store.js';
 import { legacy_mode_flag } from '../../flags/index.js';
@@ -282,7 +282,11 @@ export function prop(props, key, flags, fallback) {
 	} else {
 		prop_value = /** @type {V} */ (props[key]);
 	}
-	var setter = get_descriptor(props, key)?.set;
+
+	var setter =
+		get_descriptor(props, key)?.set ??
+		// Can be the case when someone does `mount(Component, props)` with `let props = $state({...})`
+		(STATE_SYMBOL in props ? (v) => (props[key] = v) : undefined);
 
 	var fallback_value = /** @type {V} */ (fallback);
 	var fallback_dirty = true;

packages/svelte/src/legacy/legacy-client.js

@@ -104,6 +104,20 @@ class Svelte4Component {
 				set(target, prop, value) {
 					set(sources.get(prop) ?? add_source(prop, value), value);
 					return Reflect.set(target, prop, value);
+				},
+				getOwnPropertyDescriptor(target, prop) {
+					// TODO this throws "invalid binding" errors on the component side
+					const desc = Reflect.getOwnPropertyDescriptor(target, prop);
+					if (!desc?.configurable) return desc;
+					return {
+						get: () => get(sources.get(prop) ?? add_source(prop, Reflect.get(target, prop))),
+						set: (value) => {
+							set(sources.get(prop) ?? add_source(prop, value), value);
+							return Reflect.set(target, prop, value);
+						},
+						enumerable: desc.enumerable,
+						configurable: desc.configurable
+					};
 				}
 			}
 		);