breaking: avoid flushing queued updates on mount/hydrate (#12587)

* breaking: avoid flushing queued updates on mount/hydrat

* Fix tests

* Update packages/svelte/src/internal/client/render.js

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

* tweak

* tweak

---------

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

.changeset/slow-gorillas-yawn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: avoid flushing queued updates on mount/hydrate

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

@@ -79,8 +79,7 @@ export function set_text(text, value) {
  */
 export function mount(component, options) {
 	const anchor = options.anchor ?? options.target.appendChild(empty());
-	// Don't flush previous effects to ensure order of outer effects stays consistent
-	return flush_sync(() => _mount(component, { ...options, anchor }), false);
+	return _mount(component, { ...options, anchor });
 }
 
 /**
@@ -113,8 +112,6 @@ export function hydrate(component, options) {
 	const previous_hydrate_node = hydrate_node;
 
 	try {
-		// Don't flush previous effects to ensure order of outer effects stays consistent
-		return flush_sync(() => {
 		var anchor = /** @type {import('#client').TemplateNode} */ (target.firstChild);
 		while (
 			anchor &&
@@ -141,12 +138,9 @@ export function hydrate(component, options) {
 			throw HYDRATION_ERROR;
 		}
 
-			// flush_sync will run this callback and then synchronously run any pending effects,
-			// which don't belong to the hydration phase anymore - therefore reset it here
 		set_hydrating(false);
 
-			return instance;
-		}, false);
+		return /**  @type {Exports} */ (instance);
 	} catch (error) {
 		if (error === HYDRATION_ERROR) {
 			// TODO it's possible for event listeners to have been added and

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

@@ -670,10 +670,9 @@ function process_effects(effect, collected_effects) {
  * Internal version of `flushSync` with the option to not flush previous effects.
  * Returns the result of the passed function, if given.
  * @param {() => any} [fn]
- * @param {boolean} [flush_previous]
  * @returns {any}
  */
-export function flush_sync(fn, flush_previous = true) {
+export function flush_sync(fn) {
 	var previous_scheduler_mode = current_scheduler_mode;
 	var previous_queued_root_effects = current_queued_root_effects;
 
@@ -687,9 +686,7 @@ export function flush_sync(fn, flush_previous = true) {
 		current_queued_root_effects = root_effects;
 		is_micro_task_queued = false;
 
-		if (flush_previous) {
 		flush_queued_root_effects(previous_queued_root_effects);
-		}
 
 		var result = fn?.();
 

packages/svelte/tests/hydration/test.ts

@@ -8,6 +8,7 @@ import { suite, assert_ok, type BaseTest } from '../suite.js';
 import { createClassComponent } from 'svelte/legacy';
 import { render } from 'svelte/server';
 import type { CompileOptions } from '#compiler';
+import { flushSync } from 'svelte';
 
 interface HydrationTest extends BaseTest {
 	load_compiled?: boolean;
@@ -114,6 +115,7 @@ const { test, run } = suite<HydrationTest>(async (config, cwd) => {
 
 		if (!override) {
 			const expected = read(`${cwd}/_expected.html`) ?? rendered.html;
+			flushSync();
 			assert.equal(target.innerHTML.trim(), expected.trim());
 		}
 

packages/svelte/tests/runtime-browser/driver.js

@@ -5,6 +5,7 @@ import config from '__CONFIG__';
 // @ts-expect-error
 import * as assert from 'assert.js';
 import { createClassComponent } from 'svelte/legacy';
+import { flushSync } from 'svelte';
 
 /** @param {HTMLElement} target */
 export default async function (target) {
@@ -45,6 +46,8 @@ export default async function (target) {
 			} while (new Date().getTime() <= start + ms);
 		};
 
+		flushSync();
+
 		if (config.html) {
 			assert.htmlEqual(target.innerHTML, config.html);
 		}

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input-group/_config.js

@@ -1,3 +1,4 @@
+import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -9,6 +10,7 @@ export default test({
 		inputs[1].dispatchEvent(new window.Event('change'));
 		// Hydration shouldn't reset the value to 1
 		hydrate();
+		flushSync();
 
 		assert.htmlEqual(
 			target.innerHTML,

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input/_config.js

@@ -1,3 +1,4 @@
+import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -9,6 +10,7 @@ export default test({
 		input.dispatchEvent(new window.Event('input'));
 		// Hydration shouldn't reset the value to empty
 		hydrate();
+		flushSync();
 
 		assert.htmlEqual(target.innerHTML, '<input type="text">\nfoo');
 	}