From 97587c3284f4767a7d8149abe4241aaeeb95554b Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 24 Feb 2025 13:46:39 -0500
Subject: [PATCH] WIP

---
 .../src/internal/client/reactivity/forks.js   | 12 +++++--
 .../svelte/src/internal/client/runtime.js     | 32 ++++++++++++++-----
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/forks.js b/packages/svelte/src/internal/client/reactivity/forks.js
index 18f94a8119..322c678b6c 100644
--- a/packages/svelte/src/internal/client/reactivity/forks.js
+++ b/packages/svelte/src/internal/client/reactivity/forks.js
@@ -1,4 +1,5 @@
 /** @import { Effect, Source } from '#client' */
+import { noop } from '../../shared/utils.js';
 import { DIRTY } from '../constants.js';
 import { flushSync } from '../runtime.js';
 import { internal_set, mark_reactions } from './sources.js';
@@ -26,6 +27,11 @@ export class Fork {
 	#pending = 0;
 
 	apply() {
+		if (forks.size === 1) {
+			// if this is the latest (and only) fork, we have nothing to do
+			return noop;
+		}
+
 		var values = new Map();
 
 		for (const source of this.previous.keys()) {
@@ -53,8 +59,6 @@ export class Fork {
 			for (const [source, value] of values) {
 				source.v = value;
 			}
-
-			active_fork = null;
 		};
 	}
 
@@ -119,3 +123,7 @@ export class Fork {
 		return active_fork;
 	}
 }
+
+export function remove_active_fork() {
+	active_fork = null;
+}
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 41d7810eb7..2c78e90fbe 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -49,7 +49,7 @@ import {
 import { Boundary } from './dom/blocks/boundary.js';
 import * as w from './warnings.js';
 import { is_firefox } from './dom/operations.js';
-import { active_fork, Fork } from './reactivity/forks.js';
+import { active_fork, Fork, remove_active_fork } from './reactivity/forks.js';
 import { log_effect_tree } from './dev/debug.js';
 
 // Used for DEV time error handling
@@ -670,7 +670,7 @@ function flush_queued_root_effects() {
 		return;
 	}
 
-	var revert = active_fork.apply();
+	var fork = active_fork;
 
 	try {
 		var flush_count = 0;
@@ -692,18 +692,19 @@ function flush_queued_root_effects() {
 					root.f ^= CLEAN;
 				}
 
-				process_effects(root, active_fork);
+				process_effects(root, fork);
 			}
 		}
 	} finally {
 		// TODO this doesn't seem quite right — may run into
 		// interesting cases where there are multiple roots.
 		// it'll do for now though
-		if (active_fork.settled()) {
-			active_fork.remove();
+		if (fork.settled()) {
+			fork.remove();
 		}
 
-		revert();
+		remove_active_fork();
+
 		is_flushing = false;
 
 		last_scheduled_effect = null;
@@ -787,8 +788,13 @@ export function schedule_effect(signal) {
  * @param {Fork} fork
  */
 function process_effects(effect, fork) {
+	var revert = fork.apply();
+
 	var current_effect = effect.first;
 
+	/** @type {Effect[]} */
+	var async_effects = [];
+
 	/** @type {Effect[]} */
 	var render_effects = [];
 
@@ -807,7 +813,11 @@ function process_effects(effect, fork) {
 			active_fork?.skipped_effects.has(current_effect);
 
 		if (!skip) {
-			if ((flags & (BLOCK_EFFECT | EFFECT_ASYNC)) !== 0) {
+			if ((flags & EFFECT_ASYNC) !== 0) {
+				if (check_dirtiness(current_effect)) {
+					async_effects.push(current_effect);
+				}
+			} else if ((flags & BLOCK_EFFECT) !== 0) {
 				if (check_dirtiness(current_effect)) {
 					update_effect(current_effect);
 				}
@@ -849,10 +859,16 @@ function process_effects(effect, fork) {
 		current_effect = sibling;
 	}
 
-	if (fork.settled()) {
+	if (async_effects.length === 0 && fork.settled()) {
 		flush_queued_effects(render_effects);
 		flush_queued_effects(effects);
 	}
+
+	revert();
+
+	for (const effect of async_effects) {
+		update_effect(effect);
+	}
 }
 
 /**