fix: handle asnyc updates within pending boundary (#17873)

When an async value is updated inside the boundary while the pending
snippet is shown, we previously didn't notice that update and instead
showed an outdated value once it resolved. This fixes that by rejecting
all deferreds inside an async_derived while the pending snippet is
shown.

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/nasty-friends-crash.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle asnyc updates within pending boundary

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

@@ -146,8 +146,18 @@ export function async_derived(fn, label, location) {
 		if (should_suspend) {
 			var decrement_pending = increment_pending();
 
-			deferreds.get(batch)?.reject(STALE_REACTION);
-			deferreds.delete(batch); // delete to ensure correct order in Map iteration below
+			if (/** @type {Boundary} */ (parent.b).is_rendered()) {
+				deferreds.get(batch)?.reject(STALE_REACTION);
+				deferreds.delete(batch); // delete to ensure correct order in Map iteration below
+			} else {
+				// While the boundary is still showing pending, a new run supersedes all older in-flight runs
+				// for this async expression. Cancel eagerly so resolution cannot commit stale values.
+				for (const d of deferreds.values()) {
+					d.reject(STALE_REACTION);
+				}
+				deferreds.clear();
+			}
+
 			deferreds.set(batch, d);
 		}
 

packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/_config.js

@@ -0,0 +1,51 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [shift, increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		increment.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				loading
+			`
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<button>increment</button>
+				1
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-boundary-update-while-pending/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let queue = [];
+
+	function push(value) {
+		const deferred = Promise.withResolvers();
+		queue.push(() => deferred.resolve(value));
+		return deferred.promise;
+	}
+
+	let count = $state(0);
+</script>
+
+<button onclick={() => queue.shift()()}>shift</button>
+<button onclick={() => count++}>increment</button>
+
+<svelte:boundary>
+	{await push(count)}
+	{#snippet pending()}loading{/snippet}
+</svelte:boundary>