fix: run blocks eagerly during flush (#16631)

fixes #16548

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

Author: Rich-Harris

.changeset/clever-months-clap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: run blocks eagerly during flush instead of aborting

packages/svelte/src/internal/client/dom/operations.js

@@ -6,6 +6,7 @@ import { get_descriptor, is_extensible } from '../../shared/utils.js';
 import { active_effect } from '../runtime.js';
 import { async_mode_flag } from '../../flags/index.js';
 import { TEXT_NODE, EFFECT_RAN } from '#client/constants';
+import { eager_block_effects } from '../reactivity/batch.js';
 
 // export these for reference in the compiled code, making global name deduplication unnecessary
 /** @type {Window} */
@@ -214,6 +215,7 @@ export function clear_text_content(node) {
  */
 export function should_defer_append() {
 	if (!async_mode_flag) return false;
+	if (eager_block_effects !== null) return false;
 
 	var flags = /** @type {Effect} */ (active_effect).f;
 	return (flags & EFFECT_RAN) !== 0;

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

@@ -292,12 +292,12 @@ export class Batch {
 			if (!skip && effect.fn !== null) {
 				if (is_branch) {
 					effect.f ^= CLEAN;
+				} else if ((flags & EFFECT) !== 0) {
+					this.#effects.push(effect);
+				} else if (async_mode_flag && (flags & RENDER_EFFECT) !== 0) {
+					this.#render_effects.push(effect);
 				} else if ((flags & CLEAN) === 0) {
-					if ((flags & EFFECT) !== 0) {
-						this.#effects.push(effect);
-					} else if (async_mode_flag && (flags & RENDER_EFFECT) !== 0) {
-						this.#render_effects.push(effect);
-					} else if ((flags & ASYNC) !== 0) {
+					if ((flags & ASYNC) !== 0) {
 						var effects = effect.b?.pending ? this.#boundary_async_effects : this.#async_effects;
 						effects.push(effect);
 					} else if (is_dirty(effect)) {
@@ -584,6 +584,9 @@ function infinite_loop_guard() {
 	}
 }
 
+/** @type {Effect[] | null} */
+export let eager_block_effects = null;
+
 /**
  * @param {Array<Effect>} effects
  * @returns {void}
@@ -598,7 +601,7 @@ function flush_queued_effects(effects) {
 		var effect = effects[i++];
 
 		if ((effect.f & (DESTROYED | INERT)) === 0 && is_dirty(effect)) {
-			var n = current_batch ? current_batch.current.size : 0;
+			eager_block_effects = [];
 
 			update_effect(effect);
 
@@ -619,21 +622,20 @@ function flush_queued_effects(effects) {
 				}
 			}
 
-			// if state is written in a user effect, abort and re-schedule, lest we run
-			// effects that should be removed as a result of the state change
-			if (
-				current_batch !== null &&
-				current_batch.current.size > n &&
-				(effect.f & USER_EFFECT) !== 0
-			) {
-				break;
+			if (eager_block_effects.length > 0) {
+				// TODO this feels incorrect! it gets the tests passing
+				old_values.clear();
+
+				for (const e of eager_block_effects) {
+					update_effect(e);
+				}
+
+				eager_block_effects = [];
 			}
 		}
 	}
 
-	while (i < length) {
-		schedule_effect(effects[i++]);
-	}
+	eager_block_effects = null;
 }
 
 /**

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

@@ -33,7 +33,7 @@ import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { get_stack, tag_proxy } from '../dev/tracing.js';
 import { component_context, is_runes } from '../context.js';
-import { Batch, schedule_effect } from './batch.js';
+import { Batch, eager_block_effects, schedule_effect } from './batch.js';
 import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 
@@ -334,6 +334,12 @@ function mark_reactions(signal, status) {
 		if ((flags & DERIVED) !== 0) {
 			mark_reactions(/** @type {Derived} */ (reaction), MAYBE_DIRTY);
 		} else if (not_dirty) {
+			if ((flags & BLOCK_EFFECT) !== 0) {
+				if (eager_block_effects !== null) {
+					eager_block_effects.push(/** @type {Effect} */ (reaction));
+				}
+			}
+
 			schedule_effect(/** @type {Effect} */ (reaction));
 		}
 	}

packages/svelte/tests/runtime-runes/samples/effect-loop-3/Child.svelte

@@ -0,0 +1,13 @@
+<script>
+	let { children } = $props();
+
+	let inited = $state(false);
+
+	$effect(() => {
+		inited = true;
+	});
+</script>
+
+{#if inited}
+	<span>{@render children()}</span>
+{/if}

packages/svelte/tests/runtime-runes/samples/effect-loop-3/_config.js

@@ -0,0 +1,12 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [button] = target.querySelectorAll('button');
+
+		assert.doesNotThrow(() => {
+			flushSync(() => button.click());
+		});
+	}
+});

packages/svelte/tests/runtime-runes/samples/effect-loop-3/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	import Child from './Child.svelte';
+
+	let show = $state(false);
+</script>
+
+<button onclick={() => show = !show}>
+	toggle
+</button>
+
+{#if show}
+	{#each { length: 1234 } as i}
+		<Child>{i}</Child>
+	{/each}
+{/if}