From 7aa34b4eebae3a812c24e5ae6990dd99326625a4 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Mon, 20 May 2024 22:01:01 +0100
Subject: [PATCH] breaking: delegated event handlers now yield effect updates

---
 .changeset/popular-cameras-tie.md             |  5 ++
 .../internal/client/dom/elements/events.js    |  4 ++
 .../svelte/src/internal/client/runtime.js     | 49 ++++++++++++++++++-
 3 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/popular-cameras-tie.md

diff --git a/.changeset/popular-cameras-tie.md b/.changeset/popular-cameras-tie.md
new file mode 100644
index 0000000000..7c306a8c3e
--- /dev/null
+++ b/.changeset/popular-cameras-tie.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+breaking: delegated event handlers now yield effect updates
diff --git a/packages/svelte/src/internal/client/dom/elements/events.js b/packages/svelte/src/internal/client/dom/elements/events.js
index e0298af92a..b31fe7fdcc 100644
--- a/packages/svelte/src/internal/client/dom/elements/events.js
+++ b/packages/svelte/src/internal/client/dom/elements/events.js
@@ -1,5 +1,6 @@
 import { render_effect } from '../../reactivity/effects.js';
 import { all_registered_events, root_event_handles } from '../../render.js';
+import { FLUSH_YIELD, current_scheduler_mode, set_schedule_mode } from '../../runtime.js';
 import { define_property, is_array } from '../../utils.js';
 import { hydrating } from '../hydration.js';
 
@@ -180,9 +181,12 @@ export function handle_event_propagation(handler_element, event) {
 		}
 	}
 
+	const previous_scheduler_mode = current_scheduler_mode;
 	try {
+		set_schedule_mode(FLUSH_YIELD);
 		next(current_target);
 	} finally {
+		set_schedule_mode(previous_scheduler_mode);
 		// @ts-expect-error is used above
 		event.__root = handler_element;
 		// @ts-expect-error is used above
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 43ccd4d703..935bbe3734 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -28,17 +28,27 @@ import { lifecycle_outside_component } from '../shared/errors.js';
 
 const FLUSH_MICROTASK = 0;
 const FLUSH_SYNC = 1;
+export const FLUSH_YIELD = 2;
 
 // Used for DEV time error handling
 /** @param {WeakSet<Error>} value */
 const handled_errors = new WeakSet();
 // Used for controlling the flush of effects.
-let current_scheduler_mode = FLUSH_MICROTASK;
+export let current_scheduler_mode = FLUSH_MICROTASK;
 // Used for handling scheduling
 let is_micro_task_queued = false;
+let is_yield_task_queued = false;
+
 export let is_flushing_effect = false;
 export let is_destroying_effect = false;
 
+/**
+ * @param {number} value
+ */
+export function set_schedule_mode(value) {
+	current_scheduler_mode = value;
+}
+
 /** @param {boolean} value */
 export function set_is_flushing_effect(value) {
 	is_flushing_effect = value;
@@ -521,13 +531,17 @@ function infinite_loop_guard() {
  * @returns {void}
  */
 function flush_queued_root_effects(root_effects) {
+	const length = root_effects.length;
+	if (length === 0) {
+		return;
+	}
 	infinite_loop_guard();
 
 	var previously_flushing_effect = is_flushing_effect;
 	is_flushing_effect = true;
 
 	try {
-		for (var i = 0; i < root_effects.length; i++) {
+		for (var i = 0; i < length; i++) {
 			var effect = root_effects[i];
 
 			// When working with custom elements, the root effects might not have a root
@@ -565,6 +579,7 @@ function flush_queued_effects(effects) {
 
 function process_microtask() {
 	is_micro_task_queued = false;
+	is_yield_task_queued = false;
 	if (flush_count > 101) {
 		return;
 	}
@@ -576,6 +591,31 @@ function process_microtask() {
 	}
 }
 
+const yield_task =
+	typeof requestAnimationFrame === 'undefined' ? setTimeout : requestAnimationFrame;
+
+async function process_yieldtask() {
+	// TODO: replace this with scheduler.yield when it becomes standard
+	await new Promise((fulfil) => {
+		yield_task(() => {
+			setTimeout(fulfil, 0);
+		});
+		// In case of background task
+		setTimeout(fulfil, 100);
+	});
+	is_yield_task_queued = false;
+	is_micro_task_queued = false;
+	if (flush_count > 101) {
+		return;
+	}
+	const previous_queued_root_effects = current_queued_root_effects;
+	current_queued_root_effects = [];
+	flush_queued_root_effects(previous_queued_root_effects);
+	if (!is_yield_task_queued) {
+		flush_count = 0;
+	}
+}
+
 /**
  * @param {import('#client').Effect} signal
  * @returns {void}
@@ -586,6 +626,11 @@ export function schedule_effect(signal) {
 			is_micro_task_queued = true;
 			queueMicrotask(process_microtask);
 		}
+	} else if (current_scheduler_mode === FLUSH_YIELD) {
+		if (!is_yield_task_queued) {
+			is_yield_task_queued = true;
+			process_yieldtask();
+		}
 	}
 
 	var effect = signal;