fix: keep creating state sources for property checks during component init (#18949)

.changeset/keep-has-check-sources.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep creating state sources for property checks during component init

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

@@ -2,6 +2,7 @@
 import { DEV } from 'esm-env';
 import {
 	get,
+	active_effect,
 	update_version,
 	active_reaction,
 	set_update_version,
@@ -240,7 +241,8 @@ export function proxy(value) {
 
 			if (
 				s !== undefined ||
-				(active_reaction !== null && (!has || get_descriptor(target, prop)?.writable))
+				((active_effect !== null || active_reaction !== null) &&
+					(!has || get_descriptor(target, prop)?.writable))
 			) {
 				if (s === undefined) {
 					s = with_parent(() => {

packages/svelte/tests/runtime-runes/samples/async-fork-discard-after-in-check/_config.js

@@ -0,0 +1,18 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		await tick();
+
+		const [rerun, discard] = target.querySelectorAll('button');
+
+		rerun.click();
+		await tick();
+
+		discard.click();
+		await tick();
+
+		assert.deepEqual(logs, [1, false, 1, 1, 1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-discard-after-in-check/main.svelte

@@ -0,0 +1,49 @@
+<script>
+	import { fork } from 'svelte';
+
+	const init = $state({ x: 1 });
+	const init_missing = $state({});
+	const root = $state({ x: 1 });
+	const teardown = $state({ x: 1 });
+	const after_await = $state({ x: 1 });
+
+	let count = $state(0);
+
+	// each object is first checked with `in` while an effect is active but no reaction is
+	'x' in init;
+	'y' in init_missing;
+
+	$effect.root(() => {
+		'x' in root;
+	});
+
+	$effect(() => {
+		count;
+
+		return () => {
+			'x' in teardown;
+		};
+	});
+
+	await Promise.resolve();
+
+	'x' in after_await;
+</script>
+
+<button onclick={() => count++}>rerun</button>
+
+<button
+	onclick={() => {
+		fork(() => {
+			init.x = 2;
+			init_missing.y = 1;
+			root.x = 2;
+			teardown.x = 2;
+			after_await.x = 2;
+		}).discard();
+
+		console.log(init.x, 'y' in init_missing, root.x, teardown.x, after_await.x);
+	}}
+>
+	discard
+</button>