create sources when reading proxy properties inside deriveds

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

@@ -1,4 +1,4 @@
-import { effect_active, get, set, increment, source } from './runtime.js';
+import { effect_active, get, set, increment, source, updating_derived } from './runtime.js';
 import { get_descriptor, is_array } from './utils.js';
 
 /** @typedef {{ p: StateObject | null; s: Map<string | symbol, import('./types.js').SourceSignal<any>>; v: import('./types.js').SourceSignal<number>; a: boolean }} Metadata */
@@ -73,7 +73,7 @@ const handler = {
 		// but only if it's an own property and not a prototype property
 		if (
 			s === undefined &&
-			effect_active() &&
+			(effect_active() || updating_derived) &&
 			(!(prop in target) || get_descriptor(target, prop)?.writable)
 		) {
 			s = source(wrap(target[prop], receiver));

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

@@ -87,6 +87,8 @@ export let current_block = null;
 export let current_component_context = null;
 export let is_ssr = false;
 
+export let updating_derived = false;
+
 /**
  * @param {boolean} ssr
  * @returns {void}
@@ -661,7 +663,10 @@ export async function tick() {
  * @returns {void}
  */
 function update_derived(signal, force_schedule) {
+	const previous_updating_derived = updating_derived;
+	updating_derived = true;
 	const value = execute_signal_fn(signal);
+	updating_derived = previous_updating_derived;
 	const status =
 		current_skip_consumer || (current_effect === null && (signal.f & UNOWNED) !== 0)
 			? DIRTY