fix: ensure onMount correctly fires when used externally

Author: trueadm

.changeset/early-hounds-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure onMount correctly fires when used externally

packages/svelte/src/index-client.js

@@ -25,14 +25,10 @@ export function onMount(fn) {
 		lifecycle_outside_component('onMount');
 	}
 
-	if (component_context.l !== null) {
-		init_update_callbacks(component_context).m.push(fn);
-	} else {
-		user_effect(() => {
-			const cleanup = untrack(fn);
-			if (typeof cleanup === 'function') return /** @type {() => void} */ (cleanup);
-		});
-	}
+	user_effect(() => {
+		const cleanup = untrack(fn);
+		if (typeof cleanup === 'function') return /** @type {() => void} */ (cleanup);
+	});
 }
 
 /**

packages/svelte/tests/runtime-legacy/samples/onmount-external/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs }) {
+		assert.deepEqual(logs, ['mounted']);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/onmount-external/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import { MyClass } from "./myClass.js";
+	
+	new MyClass();
+</script>

packages/svelte/tests/runtime-legacy/samples/onmount-external/myClass.js

@@ -0,0 +1,7 @@
+import { onMount } from 'svelte';
+
+export class MyClass {
+	constructor() {
+		onMount(() => console.log('mounted'));
+	}
+}