chore: speedup hydration around input and select values (#11717)

* chore: speedup hydration around input and select values

* use idle tasks to do the work

---------

Co-authored-by: Dominic Gannaway <dg@domgan.com>

.changeset/mean-jokes-exist.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+chore: speedup hydration around input and select values

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -3,10 +3,11 @@ import { hydrating } from '../hydration.js';
 import { get_descriptors, get_prototype_of, map_get, map_set } from '../../utils.js';
 import { AttributeAliases, DelegatedEvents, namespace_svg } from '../../../../constants.js';
 import { create_event, delegate } from './events.js';
-import { autofocus } from './misc.js';
+import { add_form_reset_listener, autofocus } from './misc.js';
 import { effect, effect_root } from '../../reactivity/effects.js';
 import * as w from '../../warnings.js';
 import { LOADING_ATTR_SYMBOL } from '../../constants.js';
+import { queue_idle_task } from '../task.js';
 
 /**
  * The value/checked attribute in the template actually corresponds to the defaultValue property, so we need
@@ -16,12 +17,23 @@ import { LOADING_ATTR_SYMBOL } from '../../constants.js';
  */
 export function remove_input_attr_defaults(dom) {
 	if (hydrating) {
-		// using getAttribute instead of dom.value allows us to have
-		// null instead of "on" if the user didn't set a value
-		const value = dom.getAttribute('value');
-		set_attribute(dom, 'value', null);
-		set_attribute(dom, 'checked', null);
-		if (value) dom.value = value;
+		let already_removed = false;
+		// We try and remove the default attributes later, rather than sync during hydration.
+		// Doing it sync during hydration has a negative impact on performance, but deferring the
+		// work in an idle task alleviates this greatly. If a form reset event comes in before
+		// the idle callback, then we ensure the input defaults are cleared just before.
+		const remove_defaults = () => {
+			if (already_removed) return;
+			already_removed = true;
+			const value = dom.getAttribute('value');
+			set_attribute(dom, 'value', null);
+			set_attribute(dom, 'checked', null);
+			if (value) dom.value = value;
+		};
+		// @ts-expect-error
+		dom.__on_r = remove_defaults;
+		queue_idle_task(remove_defaults);
+		add_form_reset_listener();
 	}
 }
 

packages/svelte/src/internal/client/dom/elements/bindings/shared.js

@@ -1,4 +1,5 @@
 import { render_effect } from '../../../reactivity/effects.js';
+import { add_form_reset_listener } from '../misc.js';
 
 /**
  * Fires the handler once immediately (unless corresponding arg is set to `false`),
@@ -26,8 +27,6 @@ export function listen(target, events, handler, call_handler_immediately = true)
 	});
 }
 
-let listening_to_form_reset = false;
-
 /**
  * Listen to the given event, and then instantiate a global form reset listener if not already done,
  * to notify all bindings when the form is reset
@@ -52,24 +51,5 @@ export function listen_to_event_and_reset_event(element, event, handler, on_rese
 		element.__on_r = on_reset;
 	}
 
-	if (!listening_to_form_reset) {
-		listening_to_form_reset = true;
-		document.addEventListener(
-			'reset',
-			(evt) => {
-				// Needs to happen one tick later or else the dom properties of the form
-				// elements have not updated to their reset values yet
-				Promise.resolve().then(() => {
-					if (!evt.defaultPrevented) {
-						for (const e of /**@type {HTMLFormElement} */ (evt.target).elements) {
-							// @ts-expect-error
-							e.__on_r?.();
-						}
-					}
-				});
-			},
-			// In the capture phase to guarantee we get noticed of it (no possiblity of stopPropagation)
-			{ capture: true }
-		);
-	}
+	add_form_reset_listener();
 }

packages/svelte/src/internal/client/dom/elements/bindings/this.js

@@ -1,7 +1,7 @@
 import { STATE_SYMBOL } from '../../../constants.js';
 import { effect, render_effect } from '../../../reactivity/effects.js';
 import { untrack } from '../../../runtime.js';
-import { queue_task } from '../../task.js';
+import { queue_micro_task } from '../../task.js';
 
 /**
  * @param {any} bound_value
@@ -49,7 +49,7 @@ export function bind_this(element_or_component, update, get_value, get_parts) {
 
 		return () => {
 			// We cannot use effects in the teardown phase, we we use a microtask instead.
-			queue_task(() => {
+			queue_micro_task(() => {
 				if (parts && is_bound_this(get_value(...parts), element_or_component)) {
 					update(null, ...parts);
 				}

packages/svelte/src/internal/client/dom/elements/events.js

@@ -2,7 +2,7 @@ import { render_effect } from '../../reactivity/effects.js';
 import { all_registered_events, root_event_handles } from '../../render.js';
 import { define_property, is_array } from '../../utils.js';
 import { hydrating } from '../hydration.js';
-import { queue_task } from '../task.js';
+import { queue_micro_task } from '../task.js';
 
 /**
  * SSR adds onload and onerror attributes to catch those events before the hydration.
@@ -56,7 +56,7 @@ export function create_event(event_name, dom, handler, options) {
 	// defer the attachment till after it's been appended to the document. TODO: remove this once Chrome fixes
 	// this bug.
 	if (event_name.startsWith('pointer')) {
-		queue_task(() => {
+		queue_micro_task(() => {
 			dom.addEventListener(event_name, target_handler, options);
 		});
 	} else {

packages/svelte/src/internal/client/dom/elements/misc.js

@@ -31,3 +31,28 @@ export function remove_textarea_child(dom) {
 		clear_text_content(dom);
 	}
 }
+
+let listening_to_form_reset = false;
+
+export function add_form_reset_listener() {
+	if (!listening_to_form_reset) {
+		listening_to_form_reset = true;
+		document.addEventListener(
+			'reset',
+			(evt) => {
+				// Needs to happen one tick later or else the dom properties of the form
+				// elements have not updated to their reset values yet
+				Promise.resolve().then(() => {
+					if (!evt.defaultPrevented) {
+						for (const e of /**@type {HTMLFormElement} */ (evt.target).elements) {
+							// @ts-expect-error
+							e.__on_r?.();
+						}
+					}
+				});
+			},
+			// In the capture phase to guarantee we get noticed of it (no possiblity of stopPropagation)
+			{ capture: true }
+		);
+	}
+}

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

@@ -1,33 +1,63 @@
 import { run_all } from '../../shared/utils.js';
 
-let is_task_queued = false;
+// Fallback for when requestIdleCallback is not available
+const request_idle_callback =
+	typeof requestIdleCallback === 'undefined'
+		? (/** @type {() => void} */ cb) => setTimeout(cb, 1)
+		: requestIdleCallback;
 
+let is_micro_task_queued = false;
+let is_idle_task_queued = false;
+
+/** @type {Array<() => void>} */
+let current_queued_miro_tasks = [];
 /** @type {Array<() => void>} */
-let current_queued_tasks = [];
+let current_queued_idle_tasks = [];
+
+function process_micro_tasks() {
+	is_micro_task_queued = false;
+	const tasks = current_queued_miro_tasks.slice();
+	current_queued_miro_tasks = [];
+	run_all(tasks);
+}
 
-function process_task() {
-	is_task_queued = false;
-	const tasks = current_queued_tasks.slice();
-	current_queued_tasks = [];
+function process_idle_tasks() {
+	is_idle_task_queued = false;
+	const tasks = current_queued_idle_tasks.slice();
+	current_queued_idle_tasks = [];
 	run_all(tasks);
 }
 
 /**
  * @param {() => void} fn
  */
-export function queue_task(fn) {
-	if (!is_task_queued) {
-		is_task_queued = true;
-		queueMicrotask(process_task);
+export function queue_micro_task(fn) {
+	if (!is_micro_task_queued) {
+		is_micro_task_queued = true;
+		queueMicrotask(process_micro_tasks);
+	}
+	current_queued_miro_tasks.push(fn);
+}
+
+/**
+ * @param {() => void} fn
+ */
+export function queue_idle_task(fn) {
+	if (!is_idle_task_queued) {
+		is_idle_task_queued = true;
+		request_idle_callback(process_idle_tasks);
 	}
-	current_queued_tasks.push(fn);
+	current_queued_idle_tasks.push(fn);
 }
 
 /**
  * Synchronously run any queued tasks.
  */
 export function flush_tasks() {
-	if (is_task_queued) {
-		process_task();
+	if (is_micro_task_queued) {
+		process_micro_tasks();
+	}
+	if (is_idle_task_queued) {
+		process_idle_tasks();
 	}
 }