From 18479217a736e51fa570b13cf1a4ae035c84e2ff Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 2 Apr 2026 21:52:08 -0400
Subject: [PATCH] create new active_batch concept

---
 .../svelte/src/internal/client/reactivity/batch.js | 14 ++++++++++++++
 .../src/internal/client/reactivity/deriveds.js     |  9 ++++-----
 packages/svelte/src/internal/client/runtime.js     |  3 ++-
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 4c6efad89c..47e55bb2fc 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -53,6 +53,15 @@ const batches = new Set();
 /** @type {Batch | null} */
 export let current_batch = null;
 
+/**
+ * The batch that is currently applied. May not be the same as `current_batch`, since we
+ * null that out when flushing effects in case they set state, resulting in a new
+ * batch being created. Effects always run inside an active_batch.
+ * TODO most occurrences of `current_batch` should be this
+ * @type {Batch | null}
+ **/
+export let active_batch = null;
+
 /**
  * This is needed to avoid overwriting inputs
  * @type {Batch | null}
@@ -353,6 +362,8 @@ export class Batch {
 			batch.#roots.push(...this.#roots.filter((r) => !batch.#roots.includes(r)));
 		}
 
+		active_batch = null;
+
 		if (next_batch !== null) {
 			batches.add(next_batch);
 
@@ -507,6 +518,7 @@ export class Batch {
 			is_processing = false;
 
 			current_batch = null;
+			active_batch = null;
 			batch_values = batch_cvs = batch_wvs = null;
 
 			old_values.clear();
@@ -738,6 +750,8 @@ export class Batch {
 			return;
 		}
 
+		active_batch = this;
+
 		// if there are multiple batches, we are 'time travelling' —
 		// we need to override values with the ones in this batch...
 		batch_values = new Map(this.current);
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index d574f8100a..dc88ef70eb 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -31,7 +31,7 @@ import { get_error } from '../../shared/dev.js';
 import { async_mode_flag, tracing_mode_flag } from '../../flags/index.js';
 import { component_context } from '../context.js';
 import { UNINITIALIZED } from '../../../constants.js';
-import { batch_wvs, current_batch, get_wv, set_cv } from './batch.js';
+import { current_batch, get_wv, active_batch, set_cv } from './batch.js';
 import { increment_pending, unset_context } from './async.js';
 import { deferred, noop } from '../../shared/utils.js';
 
@@ -393,10 +393,9 @@ export function update_derived(derived) {
 	set_cv(derived, cv);
 
 	if (!derived.equals(value)) {
-		batch_wvs?.set(derived, write_version);
-
-		if (current_batch !== null) {
-			current_batch.capture_derived(derived, value);
+		if (active_batch !== null) {
+			active_batch.capture_derived(derived, value);
+			active_batch.wvs.set(derived, write_version);
 		} else {
 			derived.v = value;
 			derived.wv = write_version;
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 3833193914..e278976975 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -52,6 +52,7 @@ import {
 	current_batch,
 	flushSync,
 	get_cv,
+	active_batch,
 	schedule_effect,
 	set_cv
 } from './reactivity/batch.js';
@@ -188,7 +189,7 @@ export function is_dirty(reaction) {
 			}
 		}
 
-		var wv = batch_wvs?.get(dependency) ?? dependency.wv;
+		var wv = active_batch?.wvs.get(dependency) ?? dependency.wv;
 
 		if (wv > cv) {
 			return true;