fix: properly defer document title until async work is complete

#17061 didn't properly handle the case where the title is sync but reactive and async work outside is pending. Handle this by creating a proper effect for the document title, and make sure to wait on it and flush it once ready.

Fixes #17114

Author: dummdidumm

.changeset/cold-beds-look.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: properly defer document title until async work is complete

packages/svelte/src/compiler/phases/3-transform/client/visitors/TitleElement.js

@@ -29,17 +29,16 @@ export function TitleElement(node, context) {
 		)
 	);
 
-	// Always in an $effect so it only changes the title once async work is done
+	// Make sure it only changes the title once async work is done
 	if (has_state) {
 		context.state.after_update.push(
 			b.stmt(
 				b.call(
-					'$.template_effect',
+					'$.deferred_template_effect',
 					b.arrow(memoizer.apply(), b.block([statement])),
 					memoizer.sync_values(),
 					memoizer.async_values(),
-					memoizer.blockers(),
-					b.true
+					memoizer.blockers()
 				)
 			)
 		);

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

@@ -119,6 +119,7 @@ export {
 	legacy_pre_effect_reset,
 	render_effect,
 	template_effect,
+	deferred_template_effect,
 	effect,
 	user_effect,
 	user_pre_effect

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

@@ -144,6 +144,10 @@ export class Batch {
 
 	is_fork = false;
 
+	is_deferred() {
+		return this.is_fork || this.#blocking_pending > 0;
+	}
+
 	/**
 	 *
 	 * @param {Effect[]} root_effects
@@ -172,7 +176,7 @@ export class Batch {
 			this.#resolve();
 		}
 
-		if (this.#blocking_pending > 0 || this.is_fork) {
+		if (this.is_deferred()) {
 			this.#defer_effects(target.effects);
 			this.#defer_effects(target.render_effects);
 			this.#defer_effects(target.block_effects);

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

@@ -40,7 +40,7 @@ import { DEV } from 'esm-env';
 import { define_property } from '../../shared/utils.js';
 import { get_next_sibling } from '../dom/operations.js';
 import { component_context, dev_current_component_function, dev_stack } from '../context.js';
-import { Batch, schedule_effect } from './batch.js';
+import { Batch, current_batch, schedule_effect } from './batch.js';
 import { flatten } from './async.js';
 import { without_reactive_context } from '../dom/elements/bindings/shared.js';
 
@@ -366,11 +366,29 @@ export function render_effect(fn, flags = 0) {
  * @param {Array<() => any>} sync
  * @param {Array<() => Promise<any>>} async
  * @param {Array<Promise<void>>} blockers
- * @param {boolean} defer
  */
-export function template_effect(fn, sync = [], async = [], blockers = [], defer = false) {
+export function template_effect(fn, sync = [], async = [], blockers = []) {
 	flatten(blockers, sync, async, (values) => {
-		create_effect(defer ? EFFECT : RENDER_EFFECT, () => fn(...values.map(get)), true);
+		create_effect(RENDER_EFFECT, () => fn(...values.map(get)), true);
+	});
+}
+
+/**
+ * Like `template_effect`, but with an effect which is deferred until the batch commits
+ * @param {(...expressions: any) => void | (() => void)} fn
+ * @param {Array<() => any>} sync
+ * @param {Array<() => Promise<any>>} async
+ * @param {Array<Promise<void>>} blockers
+ */
+export function deferred_template_effect(fn, sync = [], async = [], blockers = []) {
+	var batch = /** @type {Batch} */ (current_batch);
+	var is_async = async.length > 0 || blockers.length > 0;
+
+	if (is_async) batch.increment(true);
+
+	flatten(blockers, sync, async, (values) => {
+		create_effect(EFFECT, () => fn(...values.map(get)), false);
+		if (is_async) batch.decrement(true);
 	});
 }
 

packages/svelte/tests/runtime-runes/samples/async-head-title-3/Inner.svelte

@@ -0,0 +1,15 @@
+<script>
+	let { deferred, title } = $props();
+
+	function push() {
+		const d = Promise.withResolvers();
+		deferred.push(() => d.resolve());
+		return d.promise;
+	}
+</script>
+
+<svelte:head>
+	<title>{title}</title>
+</svelte:head>
+
+<p>{await push()}</p>

packages/svelte/tests/runtime-runes/samples/async-head-title-3/_config.js

@@ -0,0 +1,24 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [toggle, resolve] = target.querySelectorAll('button');
+		toggle.click();
+		await tick();
+		assert.equal(window.document.title, '');
+
+		toggle.click();
+		await tick();
+		assert.equal(window.document.title, '');
+
+		toggle.click();
+		await tick();
+		assert.equal(window.document.title, '');
+
+		resolve.click();
+		await tick();
+		await tick();
+		assert.equal(window.document.title, 'title');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-head-title-3/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	import Inner from './Inner.svelte';
+
+	let deferred = [];
+	let show = $state(false);
+</script>
+
+<button onclick={() => show = !show}>toggle</button>
+<button onclick={() => deferred.pop()()}>resolve</button>
+{#if show}
+	<Inner {deferred} title="title" />
+{/if}