Revert "breaking: avoid flushing queued updates on mount/hydrate" (#12593)

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

This reverts commit 20b879717a.

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

.changeset/slow-gorillas-yawn.md

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

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

@@ -81,7 +81,8 @@ export function set_text(text, value) {
  */
 export function mount(component, options) {
 	const anchor = options.anchor ?? options.target.appendChild(empty());
-	return _mount(component, { ...options, anchor });
+	// Don't flush previous effects to ensure order of outer effects stays consistent
+	return flush_sync(() => _mount(component, { ...options, anchor }), false);
 }
 
 /**
@@ -114,6 +115,8 @@ 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 {TemplateNode} */ (target.firstChild);
 			while (
 				anchor &&
@@ -142,7 +145,8 @@ export function hydrate(component, options) {
 
 			set_hydrating(false);
 
-		return /**  @type {Exports} */ (instance);
+			return instance;
+		}, false);
 	} 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

@@ -671,9 +671,10 @@ 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) {
+export function flush_sync(fn, flush_previous = true) {
 	var previous_scheduler_mode = current_scheduler_mode;
 	var previous_queued_root_effects = current_queued_root_effects;
 
@@ -687,7 +688,9 @@ export function flush_sync(fn) {
 		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

@@ -8,7 +8,6 @@ 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;
@@ -115,7 +114,6 @@ 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

@@ -5,7 +5,6 @@ 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) {
@@ -46,8 +45,6 @@ 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

@@ -1,4 +1,3 @@
-import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -10,7 +9,6 @@ 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

@@ -1,4 +1,3 @@
-import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
@@ -10,7 +9,6 @@ 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');
 	}