ensure immediate flushing on pending boundaries

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

@@ -699,14 +699,25 @@ export function schedule_effect(signal) {
 export function suspend() {
 	var boundary = get_pending_boundary();
 	var batch = /** @type {Batch} */ (current_batch);
+	// In case the pending snippet is shown, we want to update the UI immediately
+	// and not have the batch be blocked on async work,
+	// since the async work is happening "hidden" behind the pending snippet.
+	var ignore_async = boundary.pending;
 
 	boundary.update_pending_count(1);
-	batch.increment();
+	if (!ignore_async) batch.increment();
 
 	return function unsuspend() {
 		boundary.update_pending_count(-1);
 		batch.activate();
+		if (ignore_async) {
+			// Template could have created new effects (for example via attachments) which need to be flushed
+			Batch.enqueue(() => {
+				batch.flush();
+			});
+		} else {
 			batch.decrement();
+		}
 
 		unset_context();
 	};

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

@@ -1,5 +1,4 @@
 /** @import { Derived, Effect, Source } from '#client' */
-/** @import { Batch } from './batch.js'; */
 import { DEV } from 'esm-env';
 import {
 	ERROR_VALUE,
@@ -33,7 +32,7 @@ import { tracing_mode_flag } from '../../flags/index.js';
 import { Boundary } from '../dom/blocks/boundary.js';
 import { component_context } from '../context.js';
 import { UNINITIALIZED } from '../../../constants.js';
-import { batch_deriveds, current_batch } from './batch.js';
+import { Batch, batch_deriveds, current_batch } from './batch.js';
 import { unset_context } from './async.js';
 
 /** @type {Effect | null} */
@@ -135,10 +134,14 @@ export function async_derived(fn, location) {
 		prev = promise;
 
 		var batch = /** @type {Batch} */ (current_batch);
+		// In case the pending snippet is shown, we want to update the UI immediately
+		// and not have the batch be blocked on async work,
+		// since the async work is happening "hidden" behind the pending snippet.
+		var ignore_async = boundary.pending;
 
 		if (should_suspend) {
 			boundary.update_pending_count(1);
-			batch.increment();
+			if (!ignore_async) batch.increment();
 		}
 
 		/**
@@ -180,8 +183,15 @@ export function async_derived(fn, location) {
 
 			if (should_suspend) {
 				boundary.update_pending_count(-1);
+				if (ignore_async) {
+					// Template could have created new effects (for example via attachments) which need to be flushed
+					Batch.enqueue(() => {
+						batch.flush();
+					});
+				} else {
 					batch.decrement();
 				}
+			}
 
 			unset_context();
 		};

packages/svelte/tests/runtime-runes/samples/async-pending-flushed-immediately/_config.js

@@ -0,0 +1,75 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [increment, resolve] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>0</button>
+			<button>shift</button>
+		`
+		);
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>1</button>
+			<button>shift</button>
+		`
+		);
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>2</button>
+			<button>shift</button>
+			<p>loading...</p>
+		`
+		);
+
+		resolve.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>2</button>
+			<button>shift</button>
+			<p>2</p>
+		`
+		);
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>2</button>
+			<button>shift</button>
+			<p>2</p>
+		`
+		);
+
+		resolve.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>3</button>
+			<button>shift</button>
+			<p>3</p>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-pending-flushed-immediately/main.svelte

@@ -0,0 +1,24 @@
+<script>
+	let count = $state(0);
+
+	let deferreds = [];
+
+	function push() {
+		const deferred = Promise.withResolvers();
+		deferreds.push(deferred);
+		return deferred.promise;
+	}
+</script>
+
+<button onclick={() => count += 1}>{count}</button>
+<button onclick={() => deferreds.shift()?.resolve(count)}>shift</button>
+
+{#if count > 1}
+	<svelte:boundary>
+		<p>{await push(count)}</p>
+
+		{#snippet pending()}
+			<p>loading...</p>
+		{/snippet}
+	</svelte:boundary>
+{/if}