fix: ensure UI consistency between deferred events

.changeset/rare-apes-jog.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure UI consistency between deferred events

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

@@ -4,17 +4,18 @@ export const RENDER_EFFECT = 1 << 3;
 export const BLOCK_EFFECT = 1 << 4;
 export const BRANCH_EFFECT = 1 << 5;
 export const ROOT_EFFECT = 1 << 6;
-export const UNOWNED = 1 << 7;
-export const CLEAN = 1 << 8;
-export const DIRTY = 1 << 9;
-export const MAYBE_DIRTY = 1 << 10;
-export const INERT = 1 << 11;
-export const DESTROYED = 1 << 12;
-export const EFFECT_RAN = 1 << 13;
+export const ACTION_EFFECT = 1 << 7;
+export const UNOWNED = 1 << 8;
+export const CLEAN = 1 << 9;
+export const DIRTY = 1 << 10;
+export const MAYBE_DIRTY = 1 << 11;
+export const INERT = 1 << 12;
+export const DESTROYED = 1 << 13;
+export const EFFECT_RAN = 1 << 14;
 /** 'Transparent' effects do not create a transition boundary */
-export const EFFECT_TRANSPARENT = 1 << 14;
+export const EFFECT_TRANSPARENT = 1 << 15;
 /** Svelte 4 legacy mode props need to be handled with deriveds and be recognized elsewhere, hence the dedicated flag */
-export const LEGACY_DERIVED_PROP = 1 << 15;
+export const LEGACY_DERIVED_PROP = 1 << 16;
 
 export const STATE_SYMBOL = Symbol('$state');
 export const LOADING_ATTR_SYMBOL = Symbol('');

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

@@ -1,3 +1,4 @@
+import { ACTION_EFFECT } from '../../constants.js';
 import { effect, render_effect } from '../../reactivity/effects.js';
 import { deep_read_state, untrack } from '../../runtime.js';
 
@@ -15,7 +16,7 @@ export function action(dom, action, get_value) {
 		if (get_value && payload?.update) {
 			var inited = false;
 
-			render_effect(() => {
+			var signal = render_effect(() => {
 				var value = get_value();
 
 				// Action's update method is coarse-grained, i.e. when anything in the passed value changes, update.
@@ -27,6 +28,7 @@ export function action(dom, action, get_value) {
 					/** @type {Function} */ (payload.update)(value);
 				}
 			});
+			signal.f |= ACTION_EFFECT;
 
 			inited = true;
 		}

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

@@ -16,7 +16,8 @@ import {
 	STATE_SYMBOL,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	LEGACY_DERIVED_PROP
+	LEGACY_DERIVED_PROP,
+	ACTION_EFFECT
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { add_owner } from './dev/ownership.js';
@@ -600,7 +601,11 @@ async function yield_tick() {
  * @returns {void}
  */
 export function schedule_effect(signal) {
-	if (current_scheduler_mode === FLUSH_MICROTASK) {
+	// If we have an action effect then we need to dispatch a standard microtask update,
+	// that's because actions might need to update the UI or attach/remove event listeners
+	// and deferring this can lead to hard-to-find bugs. If there are any deferred updates
+	// already, they'll be handled by the microtask instead.
+	if (current_scheduler_mode === FLUSH_MICROTASK || (signal.f & ACTION_EFFECT) !== 0) {
 		if (!is_micro_task_queued) {
 			is_micro_task_queued = true;
 			queueMicrotask(process_deferred);
@@ -713,6 +718,13 @@ function process_effects(effect, collected_effects) {
  */
 export function yield_updates(fn) {
 	const previous_scheduler_mode = current_scheduler_mode;
+	// If we're calling yield_updates, and we've already yielded some updates – then it's likely
+	// that the event might might try and read from the UI. In order for this to be glitch free,
+	// we can flush any changes, forcing the UI to be up-to-date, so any reads from the UI work as
+	// expected.
+	if (previous_scheduler_mode !== FLUSH_YIELD && is_yield_task_queued && !is_micro_task_queued) {
+		flush_sync();
+	}
 	try {
 		current_scheduler_mode = FLUSH_YIELD;
 		return fn();
@@ -740,6 +752,8 @@ export function flush_sync(fn, flush_previous = true) {
 
 		current_scheduler_mode = FLUSH_SYNC;
 		current_queued_root_effects = root_effects;
+		is_yield_task_queued = false;
+		is_micro_task_queued = false;
 
 		if (flush_previous) {
 			flush_queued_root_effects(previous_queued_root_effects);

packages/svelte/tests/runtime-runes/samples/deferred-events-consistency-2/_config.js

@@ -0,0 +1,12 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		const [b1] = target.querySelectorAll('button');
+		b1.click();
+		flushSync();
+
+		assert.deepEqual(logs, ['http://localhost:3000/new%20url']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/deferred-events-consistency-2/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let action = $state('old url');
+</script>
+
+<form action={action} onsubmit={function(event) {
+	console.log(this.action);
+	event.preventDefault();
+}}>
+	<button type="submit" onclick={() => {
+		action = 'new url';
+	}}>Submit</button>
+</form>

packages/svelte/tests/runtime-runes/samples/deferred-events-consistency/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, logs }) {
+		const [b1] = target.querySelectorAll('button');
+		b1.dispatchEvent(new MouseEvent('mouseup', { bubbles: true }));
+		b1.click();
+		flushSync();
+
+		assert.deepEqual(logs, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/deferred-events-consistency/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let disabled = $state(false);
+</script>
+
+<button disabled={disabled} onmouseup={() => {
+	disabled = true;
+}} onclick={() => {
+	console.log('I should not be invoked');
+}}>Click me!</button>