From 2903ba9d8f839d7e4626c5449895961085a8f1de Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 3 Jul 2026 15:39:16 +0200
Subject: [PATCH] fork fix

---
 .../src/internal/client/dom/blocks/portal.js  | 27 ++++----
 .../samples/async-portal-fork/_config.js      | 19 ++++++
 .../samples/async-portal-fork/main.svelte     | 20 ++++++
 .../async-portal-key-changing-fork/_config.js | 63 +++++++++++++++++++
 .../main.svelte                               | 35 +++++++++++
 5 files changed, 152 insertions(+), 12 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-portal-fork/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-portal-fork/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/main.svelte

diff --git a/packages/svelte/src/internal/client/dom/blocks/portal.js b/packages/svelte/src/internal/client/dom/blocks/portal.js
index c0d73687b2..c690866300 100644
--- a/packages/svelte/src/internal/client/dom/blocks/portal.js
+++ b/packages/svelte/src/internal/client/dom/blocks/portal.js
@@ -42,11 +42,17 @@ import { queue_micro_task } from '../task.js';
 
 /**
  * All known outlets (and portals waiting for outlets) for a given key.
- * `pending` is only used during hydration — it contains render functions of
- * `{#portal ...}` blocks that were created before their outlet, so that the
- * outlet can have them claim their server-rendered content upon initialisation.
+ * - `outlets` is the reactive source portals depend on
+ * - `all` is the canonical registry across all 'worlds' (in-flight batches and
+ *   forks). It is the base for the read-modify-writes to `outlets`: outlets add
+ *   and remove themselves as their effects are created and destroyed, so every
+ *   write carries the full union and registrations of other batches are never
+ *   lost. Batch-aware reads of `outlets` still scope each batch to its own view.
+ * - `pending` it contains render functions of `{#portal ...}` blocks that were
+ *   created before their outlet, so that content is rendered upon initialisation.
  * @typedef {{
  *   outlets: Source<Outlet[]>,
+ *   all: Outlet[],
  *   pending: Set<(outlet: Outlet) => void>
  * }} OutletEntry
  */
@@ -68,7 +74,7 @@ function get_outlet_entry(key) {
 	let entry = outlet_map.get(key);
 
 	if (entry === undefined) {
-		entry = { outlets: source([]), pending: new Set() };
+		entry = { outlets: source([]), all: [], pending: new Set() };
 		outlet_map.set(key, entry);
 	}
 
@@ -151,19 +157,15 @@ export function portal_outlet(node, get_id) {
 
 		const entry = get_outlet_entry(id);
 
-		internal_set(entry.outlets, [...entry.outlets.v, outlet]);
+		internal_set(entry.outlets, (entry.all = [...entry.all, outlet]));
 
-		// During hydration, portals that were created before this outlet claim
-		// their server-rendered content now, while the hydration position is known.
+		// portals that were created before this outlet claim their server-rendered content now
 		for (const render of entry.pending) {
 			render(outlet);
 		}
 
 		const unregister = () => {
-			internal_set(
-				entry.outlets,
-				entry.outlets.v.filter((o) => o !== outlet)
-			);
+			internal_set(entry.outlets, (entry.all = entry.all.filter((o) => o !== outlet)));
 		};
 
 		return () => {
@@ -503,6 +505,7 @@ export function portal(get_target, content) {
 		}
 
 		for (const k of keys) {
+			// Do the get outside the below if-block so we're notified of changes to any outlets source
 			var outlets = get(get_outlet_entry(k).outlets);
 
 			if (k === key) {
@@ -516,7 +519,7 @@ export function portal(get_target, content) {
 			if (target instanceof Element) {
 				targets.add(target);
 			} else if (hydrating) {
-				// an outlet with our key may appear later during this hydration
+				// An outlet with our key may appear later during this hydration
 				// pass (`{#portal ...}` before `{@portal ...}` in the markup).
 				// Register a callback so it can have us claim our server-rendered
 				// content at its position.
diff --git a/packages/svelte/tests/runtime-runes/samples/async-portal-fork/_config.js b/packages/svelte/tests/runtime-runes/samples/async-portal-fork/_config.js
new file mode 100644
index 0000000000..ea50dee816
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-portal-fork/_config.js
@@ -0,0 +1,19 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	solo: true,
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		assert.htmlEqual(target.innerHTML, `<button>fork</button>`);
+
+		btn?.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>fork</button> <div>portaled</div> <div>portaled</div>`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-portal-fork/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-portal-fork/main.svelte
new file mode 100644
index 0000000000..adc831da90
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-portal-fork/main.svelte
@@ -0,0 +1,20 @@
+<script>
+	import { fork } from 'svelte';
+
+	let show = $state(false);
+</script>
+
+<button onclick={async () => {
+	fork(() => {
+		show = true;
+	}).commit();
+}}>fork</button>
+
+{#portal 'target'}
+	portaled
+{/portal}
+
+{#if show}
+	<div>{@portal 'target'}</div>
+	<div>{@portal 'target'}</div>
+{/if}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/_config.js b/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/_config.js
new file mode 100644
index 0000000000..f0d0e04100
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/_config.js
@@ -0,0 +1,63 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		const [togglePortalKey, toggleOutletKey, shift, pop, commit] =
+			target.querySelectorAll('button');
+
+		togglePortalKey.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> ab'
+		);
+
+		toggleOutletKey.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> ab'
+		);
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> bb hi'
+		);
+
+		shift.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> bb hi'
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> ba'
+		);
+
+		toggleOutletKey.click();
+		await tick();
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> ba'
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle portalKey</button> <button>toggle outletKey</button> <button>shift</button> <button>pop</button> <button>commit</button> bb hi'
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/main.svelte
new file mode 100644
index 0000000000..d90b739f30
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-portal-key-changing-fork/main.svelte
@@ -0,0 +1,35 @@
+<script>
+	import { fork } from "svelte";
+
+	let portalKey = $state('a');
+	let outletKey = $state('b');
+
+	let queued = [];
+	let first = true;
+	let f;
+
+	function push(v) {
+		if (first) {
+			first = false;
+			return v;
+		}
+
+		return new Promise((resolve) => {
+			queued.push(() => resolve(v));
+		});
+	}
+</script>
+
+<button onclick={() => portalKey = portalKey === 'a' ? 'b' : 'a'}>toggle portalKey</button>
+<button onclick={() => f = fork(() => outletKey = outletKey === 'a' ? 'b' : 'a')}>toggle outletKey</button>
+<button onclick={() => queued.shift()?.()}>shift</button>
+<button onclick={() => queued.pop()?.()}>pop</button>
+<button onclick={() => f.commit()}>commit</button>
+
+{await push(portalKey + outletKey)}
+
+{#portal portalKey}
+	hi
+{/portal}
+
+{@portal outletKey}