apparently we no longer need the merging logic? we can simplify and fix stuff by removing it

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

@@ -160,61 +160,23 @@ export class Batch {
 			this.#process_root(root);
 		}
 
+		// if we didn't start any new async work, and no async work
+		// is outstanding from a previous flush, commit
 		if (this.#async_effects.length === 0 && this.#pending === 0) {
-			var merged = false;
+			var render_effects = this.#render_effects;
+			var effects = this.#effects;
 
-			// if there are older batches with overlapping
-			// state, we can't commit this batch. instead,
-			// we merge it into the older batches
-			for (const batch of batches) {
-				if (batch === this) break;
-
-				for (const [source] of batch.#current) {
-					if (this.#current.has(source)) {
-						merged = true;
-
-						for (const [source, value] of this.#current) {
-							batch.#current.set(source, value);
-						}
-
-						for (const e of this.#render_effects) {
-							set_signal_status(e, CLEAN);
-							batch.#render_effects.push(e);
-						}
-
-						for (const e of this.#effects) {
-							set_signal_status(e, CLEAN);
-							batch.#effects.push(e);
-						}
-
-						for (const e of this.skipped_effects) {
-							batch.skipped_effects.add(e);
-						}
-
-						for (const fn of this.#callbacks) {
-							batch.#callbacks.add(fn);
-						}
-
-						break;
-					}
-				}
-			}
-
-			if (!merged) {
-				var render_effects = this.#render_effects;
-				var effects = this.#effects;
-
-				this.#render_effects = [];
-				this.#effects = [];
+			this.#render_effects = [];
+			this.#effects = [];
 
-				this.#commit();
+			this.#commit();
 
-				flush_queued_effects(render_effects);
-				flush_queued_effects(effects);
+			flush_queued_effects(render_effects);
+			flush_queued_effects(effects);
 
-				this.#deferred?.resolve();
-			}
+			this.#deferred?.resolve();
 		} else {
+			// otherwise mark effects clean so they get scheduled on the next run
 			for (const e of this.#render_effects) set_signal_status(e, CLEAN);
 			for (const e of this.#effects) set_signal_status(e, CLEAN);
 		}

packages/svelte/tests/runtime-runes/samples/async-unresolved-promise/_config.js

@@ -0,0 +1,32 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [increment] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>0</p>
+			`
+		);
+
+		increment.click();
+		await tick();
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<p>2</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-unresolved-promise/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let count = $state(0);
+</script>
+
+<button onclick={() => count += 1}>
+	increment
+</button>
+
+<svelte:boundary>
+	{#if count % 2}
+		<p>{await new Promise(() => {})}</p>
+	{:else}
+		<p>{await count}</p>
+	{/if}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>