breaking: avoid flushing queued updates on mount/hydrate

.changeset/slow-gorillas-yawn.md

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

documentation/docs/04-runtime/04-imperative-component-api.md

@@ -29,6 +29,8 @@ const app = mount(App, {
 
 You can mount multiple components per page, and you can also mount from within your application, for example when creating a tooltip component and attaching it to the hovered element.
 
+Note that unlike calling `new App(...)` in Svelte 4, things like effects (including `onMount` callbacks, and action functions) will not run during `mount`. If you need to force pending effects to run (in the context of a test, for example) you can do so with `flushSync()`.
+
 ## `unmount`
 
 Unmounts a component created with [`mount`](#mount) or [`hydrate`](#hydrate):
@@ -74,3 +76,5 @@ const app = hydrate(App, {
 	props: { some: 'property' }
 });
 ```
+
+As with `mount`, effects will not run during `hydrate` — use `flushSync()` immediately afterwards if you need them to.

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

@@ -81,8 +81,7 @@ 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 });
 }
 
 /**
@@ -115,38 +114,35 @@ 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 &&
-				(anchor.nodeType !== 8 || /** @type {Comment} */ (anchor).data !== HYDRATION_START)
-			) {
-				anchor = /** @type {TemplateNode} */ (anchor.nextSibling);
-			}
+		var anchor = /** @type {TemplateNode} */ (target.firstChild);
+		while (
+			anchor &&
+			(anchor.nodeType !== 8 || /** @type {Comment} */ (anchor).data !== HYDRATION_START)
+		) {
+			anchor = /** @type {TemplateNode} */ (anchor.nextSibling);
+		}
 
-			if (!anchor) {
-				throw HYDRATION_ERROR;
-			}
+		if (!anchor) {
+			throw HYDRATION_ERROR;
+		}
 
-			set_hydrating(true);
-			set_hydrate_node(/** @type {Comment} */ (anchor));
-			hydrate_next();
+		set_hydrating(true);
+		set_hydrate_node(/** @type {Comment} */ (anchor));
+		hydrate_next();
 
-			const instance = _mount(component, { ...options, anchor });
+		const instance = _mount(component, { ...options, anchor });
 
-			if (
-				hydrate_node.nodeType !== 8 ||
-				/** @type {Comment} */ (hydrate_node).data !== HYDRATION_END
-			) {
-				w.hydration_mismatch();
-				throw HYDRATION_ERROR;
-			}
+		if (
+			hydrate_node.nodeType !== 8 ||
+			/** @type {Comment} */ (hydrate_node).data !== HYDRATION_END
+		) {
+			w.hydration_mismatch();
+			throw HYDRATION_ERROR;
+		}
 
-			set_hydrating(false);
+		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

@@ -671,10 +671,9 @@ 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;
 
@@ -688,9 +687,7 @@ 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);
-		}
+		flush_queued_root_effects(previous_queued_root_effects);
 
 		var result = fn?.();
 

packages/svelte/src/legacy/legacy-client.js

@@ -2,7 +2,7 @@
 import { mutable_source, set } from '../internal/client/reactivity/sources.js';
 import { user_pre_effect } from '../internal/client/reactivity/effects.js';
 import { hydrate, mount, unmount } from '../internal/client/render.js';
-import { get } from '../internal/client/runtime.js';
+import { flush_sync, get } from '../internal/client/runtime.js';
 import { define_property } from '../internal/shared/utils.js';
 
 /**
@@ -110,6 +110,8 @@ class Svelte4Component {
 			recover: options.recover
 		});
 
+		flush_sync();
+
 		this.#events = props.$$events;
 
 		for (const key of Object.keys(this.#instance)) {

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');
 	}

sites/svelte-5-preview/src/routes/docs/content/01-api/05-imports.md

@@ -44,6 +44,8 @@ const app = mount(App, {
 });
 ```
 
+Note that unlike calling `new App(...)` in Svelte 4, things like effects (including `onMount` callbacks, and action functions) will not run during `mount`. If you need to force pending effects to run (in the context of a test, for example) you can do so with `flushSync()`.
+
 ### `hydrate`
 
 Like `mount`, but will reuse up any HTML rendered by Svelte's SSR output (from the [`render`](#svelte-server-render) function) inside the target and make it interactive:
@@ -59,6 +61,8 @@ const app = hydrate(App, {
 });
 ```
 
+As with `mount`, effects will not run during `hydrate` — use `flushSync()` immediately afterwards if you need them to.
+
 ### `unmount`
 
 Unmounts a component created with [`mount`](#svelte-mount) or [`hydrate`](#svelte-hydrate):