From a9ac217b8f11b22adc570115dabc8f9b6a561993 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 10:34:39 -0400
Subject: [PATCH 01/26] WIP

---
 .../internal/client/dom/blocks/branches.js    | 103 ++++++++++++++++++
 .../src/internal/client/dom/blocks/if.js      |  68 +-----------
 2 files changed, 107 insertions(+), 64 deletions(-)
 create mode 100644 packages/svelte/src/internal/client/dom/blocks/branches.js

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
new file mode 100644
index 000000000000..e79e34a06809
--- /dev/null
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -0,0 +1,103 @@
+/** @import { Effect, TemplateNode } from '#client' */
+import { Batch, current_batch } from '../../reactivity/batch.js';
+import { branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
+import { create_text, should_defer_append } from '../operations.js';
+
+/**
+ * @typedef {{ effect: Effect, fragment: DocumentFragment }} Branch
+ */
+
+/**
+ * @template Key
+ */
+export class BranchManager {
+	/** @type {TemplateNode} */
+	#anchor;
+
+	/** @type {Map<Batch, Key>} */
+	#batches = new Map();
+
+	/** @type {Map<Key, Effect>} */
+	#onscreen = new Map();
+
+	/** @type {Map<Key, Branch>} */
+	#offscreen = new Map();
+
+	/**
+	 *
+	 * @param {TemplateNode} anchor
+	 */
+	constructor(anchor) {
+		this.#anchor = anchor;
+	}
+
+	#commit = () => {
+		var batch = /** @type {Batch} */ (current_batch);
+		var key = /** @type {Key} */ (this.#batches.get(batch));
+
+		var onscreen = this.#onscreen.get(key);
+
+		if (onscreen) {
+			// effect is already in the DOM — abort any current outro
+			resume_effect(onscreen);
+		} else {
+			// effect is currently offscreen. put it in the DOM
+			var offscreen = this.#offscreen.get(key);
+			if (!offscreen) throw new Error('This should never happen!');
+
+			this.#onscreen.set(key, offscreen.effect);
+			this.#offscreen.delete(key);
+
+			// remove the anchor...
+			/** @type {TemplateNode} */ (offscreen.fragment.lastChild).remove();
+
+			// ...and append the fragment
+			this.#anchor.before(offscreen.fragment);
+		}
+
+		this.#batches.delete(batch);
+
+		for (const [k, e] of this.#onscreen) {
+			if (k === key) continue;
+
+			pause_effect(e, () => {
+				// TODO if this needed by a pending batch, move the effect to
+				// a fragment and put it on this.#offscreen
+
+				this.#onscreen.delete(k);
+			});
+		}
+	};
+
+	/**
+	 *
+	 * @param {any} key
+	 * @param {(target: TemplateNode) => void} fn
+	 */
+	ensure(key, fn) {
+		if (should_defer_append()) {
+			var batch = /** @type {Batch} */ (current_batch);
+
+			if (!this.#onscreen.has(key) && !this.#offscreen.has(key)) {
+				var fragment = document.createDocumentFragment();
+				var target = create_text();
+
+				fragment.append(target);
+
+				this.#offscreen.set(key, {
+					effect: branch(() => fn(target)),
+					fragment
+				});
+			}
+
+			this.#batches.set(batch, key);
+
+			batch.add_callback(this.#commit);
+		} else {
+			this.#onscreen.set(
+				key,
+				branch(() => fn(this.#anchor))
+			);
+		}
+	}
+}
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 6349ab839931..ba0d2a6d963e 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -14,6 +14,8 @@ import { block, branch, pause_effect, resume_effect } from '../../reactivity/eff
 import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
 import { create_text, should_defer_append } from '../operations.js';
 import { current_batch } from '../../reactivity/batch.js';
+import { BranchManager } from './branches.js';
+import { noop } from '../../../shared/utils.js';
 
 // TODO reinstate https://github.com/sveltejs/svelte/pull/15250
 
@@ -30,12 +32,6 @@ export function if_block(node, fn, elseif = false) {
 
 	var anchor = node;
 
-	/** @type {Effect | null} */
-	var consequent_effect = null;
-
-	/** @type {Effect | null} */
-	var alternate_effect = null;
-
 	/** @type {typeof UNINITIALIZED | boolean | null} */
 	var condition = UNINITIALIZED;
 
@@ -48,42 +44,12 @@ export function if_block(node, fn, elseif = false) {
 		update_branch(flag, fn);
 	};
 
-	/** @type {DocumentFragment | null} */
-	var offscreen_fragment = null;
-
-	function commit() {
-		if (offscreen_fragment !== null) {
-			// remove the anchor
-			/** @type {Text} */ (offscreen_fragment.lastChild).remove();
-
-			anchor.before(offscreen_fragment);
-			offscreen_fragment = null;
-		}
-
-		var active = condition ? consequent_effect : alternate_effect;
-		var inactive = condition ? alternate_effect : consequent_effect;
-
-		if (active) {
-			resume_effect(active);
-		}
-
-		if (inactive) {
-			pause_effect(inactive, () => {
-				if (condition) {
-					alternate_effect = null;
-				} else {
-					consequent_effect = null;
-				}
-			});
-		}
-	}
+	var branches = new BranchManager(anchor);
 
 	const update_branch = (
 		/** @type {boolean | null} */ new_condition,
 		/** @type {null | ((anchor: Node) => void)} */ fn
 	) => {
-		if (condition === (condition = new_condition)) return;
-
 		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
 		let mismatch = false;
 
@@ -101,33 +67,7 @@ export function if_block(node, fn, elseif = false) {
 			}
 		}
 
-		var defer = should_defer_append();
-		var target = anchor;
-
-		if (defer) {
-			offscreen_fragment = document.createDocumentFragment();
-			offscreen_fragment.append((target = create_text()));
-		}
-
-		if (condition) {
-			consequent_effect ??= fn && branch(() => fn(target));
-		} else {
-			alternate_effect ??= fn && branch(() => fn(target));
-		}
-
-		if (defer) {
-			var batch = /** @type {Batch} */ (current_batch);
-
-			var active = condition ? consequent_effect : alternate_effect;
-			var inactive = condition ? alternate_effect : consequent_effect;
-
-			if (active) batch.skipped_effects.delete(active);
-			if (inactive) batch.skipped_effects.add(inactive);
-
-			batch.add_callback(commit);
-		} else {
-			commit();
-		}
+		branches.ensure(new_condition, fn ?? noop);
 
 		if (mismatch) {
 			// continue in hydration mode

From 39ea5e4c343a055b3f6cda40186b1c8cd06d75a6 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 11:18:47 -0400
Subject: [PATCH 02/26] WIP

---
 .../internal/client/dom/blocks/boundary.js    | 26 ++++----------
 .../internal/client/dom/blocks/branches.js    | 35 +++++++++++++++----
 .../src/internal/client/dom/blocks/if.js      |  7 ++--
 .../src/internal/client/reactivity/effects.js | 22 ++++++++++--
 4 files changed, 57 insertions(+), 33 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index ca6437e3841f..026ffb36fc51 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -8,7 +8,13 @@ import {
 import { HYDRATION_START_ELSE } from '../../../../constants.js';
 import { component_context, set_component_context } from '../../context.js';
 import { handle_error, invoke_error_boundary } from '../../error-handling.js';
-import { block, branch, destroy_effect, pause_effect } from '../../reactivity/effects.js';
+import {
+	block,
+	branch,
+	destroy_effect,
+	move_effect,
+	pause_effect
+} from '../../reactivity/effects.js';
 import {
 	active_effect,
 	active_reaction,
@@ -425,24 +431,6 @@ export class Boundary {
 	}
 }
 
-/**
- *
- * @param {Effect} effect
- * @param {DocumentFragment} fragment
- */
-function move_effect(effect, fragment) {
-	var node = effect.nodes_start;
-	var end = effect.nodes_end;
-
-	while (node !== null) {
-		/** @type {TemplateNode | null} */
-		var next = node === end ? null : /** @type {TemplateNode} */ (get_next_sibling(node));
-
-		fragment.append(node);
-		node = next;
-	}
-}
-
 export function get_boundary() {
 	return /** @type {Boundary} */ (/** @type {Effect} */ (active_effect).b);
 }
diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index e79e34a06809..9adb3ddec93a 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -1,6 +1,12 @@
 /** @import { Effect, TemplateNode } from '#client' */
 import { Batch, current_batch } from '../../reactivity/batch.js';
-import { branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
+import {
+	branch,
+	destroy_effect,
+	move_effect,
+	pause_effect,
+	resume_effect
+} from '../../reactivity/effects.js';
 import { create_text, should_defer_append } from '../operations.js';
 
 /**
@@ -57,15 +63,30 @@ export class BranchManager {
 
 		this.#batches.delete(batch);
 
-		for (const [k, e] of this.#onscreen) {
+		for (const [k, effect] of this.#onscreen) {
 			if (k === key) continue;
 
-			pause_effect(e, () => {
-				// TODO if this needed by a pending batch, move the effect to
-				// a fragment and put it on this.#offscreen
+			pause_effect(
+				effect,
+				() => {
+					const keys = Array.from(this.#batches.values());
 
-				this.#onscreen.delete(k);
-			});
+					if (keys.includes(k)) {
+						// keep the effect offscreen, as another batch will need it
+						var fragment = document.createDocumentFragment();
+						move_effect(effect, fragment);
+
+						fragment.append(create_text()); // TODO can we avoid this?
+
+						this.#offscreen.set(k, { effect, fragment });
+					} else {
+						destroy_effect(effect);
+					}
+
+					this.#onscreen.delete(k);
+				},
+				false
+			);
 		}
 	};
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index ba0d2a6d963e..23a4555e5bac 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -1,5 +1,4 @@
-/** @import { Effect, TemplateNode } from '#client' */
-/** @import { Batch } from '../../reactivity/batch.js'; */
+/** @import { TemplateNode } from '#client' */
 import { EFFECT_TRANSPARENT } from '#client/constants';
 import {
 	hydrate_next,
@@ -10,10 +9,8 @@ import {
 	set_hydrate_node,
 	set_hydrating
 } from '../hydration.js';
-import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
+import { block } from '../../reactivity/effects.js';
 import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
-import { create_text, should_defer_append } from '../operations.js';
-import { current_batch } from '../../reactivity/batch.js';
 import { BranchManager } from './branches.js';
 import { noop } from '../../../shared/utils.js';
 
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 2c9e4db911aa..bfbb95a8db7c 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -553,15 +553,16 @@ export function unlink_effect(effect) {
  * A paused effect does not update, and the DOM subtree becomes inert.
  * @param {Effect} effect
  * @param {() => void} [callback]
+ * @param {boolean} [destroy]
  */
-export function pause_effect(effect, callback) {
+export function pause_effect(effect, callback, destroy = true) {
 	/** @type {TransitionManager[]} */
 	var transitions = [];
 
 	pause_children(effect, transitions, true);
 
 	run_out_transitions(transitions, () => {
-		destroy_effect(effect);
+		if (destroy) destroy_effect(effect);
 		if (callback) callback();
 	});
 }
@@ -662,3 +663,20 @@ function resume_children(effect, local) {
 export function aborted(effect = /** @type {Effect} */ (active_effect)) {
 	return (effect.f & DESTROYED) !== 0;
 }
+
+/**
+ * @param {Effect} effect
+ * @param {DocumentFragment} fragment
+ */
+export function move_effect(effect, fragment) {
+	var node = effect.nodes_start;
+	var end = effect.nodes_end;
+
+	while (node !== null) {
+		/** @type {TemplateNode | null} */
+		var next = node === end ? null : /** @type {TemplateNode} */ (get_next_sibling(node));
+
+		fragment.append(node);
+		node = next;
+	}
+}

From a168ecdd5bee5925790d6c3003a5143d72f6d13f Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 11:35:02 -0400
Subject: [PATCH 03/26] WIP

---
 .../internal/client/dom/blocks/branches.js    | 30 +++++++++----------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 9adb3ddec93a..6b88f39a8053 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -96,29 +96,27 @@ export class BranchManager {
 	 * @param {(target: TemplateNode) => void} fn
 	 */
 	ensure(key, fn) {
-		if (should_defer_append()) {
-			var batch = /** @type {Batch} */ (current_batch);
+		var batch = /** @type {Batch} */ (current_batch);
 
-			if (!this.#onscreen.has(key) && !this.#offscreen.has(key)) {
-				var fragment = document.createDocumentFragment();
-				var target = create_text();
+		if (!this.#onscreen.has(key) && !this.#offscreen.has(key)) {
+			var fragment = document.createDocumentFragment();
+			var target = create_text();
 
-				fragment.append(target);
+			fragment.append(target);
 
-				this.#offscreen.set(key, {
-					effect: branch(() => fn(target)),
-					fragment
-				});
-			}
+			this.#offscreen.set(key, {
+				effect: branch(() => fn(target)),
+				fragment
+			});
+		}
 
-			this.#batches.set(batch, key);
+		this.#batches.set(batch, key);
 
+		// TODO in the no-defer case, we could skip the offscreen step
+		if (should_defer_append()) {
 			batch.add_callback(this.#commit);
 		} else {
-			this.#onscreen.set(
-				key,
-				branch(() => fn(this.#anchor))
-			);
+			this.#commit();
 		}
 	}
 }

From 65f597029ba4b9c8012498fb656a2d4a4a3f655e Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 11:48:36 -0400
Subject: [PATCH 04/26] WIP

---
 .../src/internal/client/dom/blocks/branches.js   | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 6b88f39a8053..15d3f0985b11 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -114,6 +114,22 @@ export class BranchManager {
 
 		// TODO in the no-defer case, we could skip the offscreen step
 		if (should_defer_append()) {
+			for (const [k, effect] of this.#onscreen) {
+				if (k === key) {
+					batch.skipped_effects.delete(effect);
+				} else {
+					batch.skipped_effects.add(effect);
+				}
+			}
+
+			for (const [k, branch] of this.#offscreen) {
+				if (k === key) {
+					batch.skipped_effects.delete(branch.effect);
+				} else {
+					batch.skipped_effects.add(branch.effect);
+				}
+			}
+
 			batch.add_callback(this.#commit);
 		} else {
 			this.#commit();

From f3fc4438acbcaddefcae28162a3dddfc2727a441 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 14:19:29 -0400
Subject: [PATCH 05/26] WIP

---
 .../phases/3-transform/client/transform-client.js |  4 +++-
 .../3-transform/client/visitors/Fragment.js       |  6 +++++-
 .../src/internal/client/reactivity/async.js       | 15 ++++++++++-----
 3 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 2629379f63d9..2431ae9f3e68 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -384,7 +384,9 @@ export function client_component(analysis, options) {
 			.../** @type {ESTree.Statement[]} */ (template.body)
 		]);
 
-		component_block.body.push(b.stmt(b.call(`$.async_body`, b.arrow([], body, true))));
+		component_block.body.push(
+			b.stmt(b.call(`$.async_body`, b.id('$$anchor'), b.arrow([b.id('$$anchor')], body, true)))
+		);
 	} else {
 		component_block.body.push(
 			...state.instance_level_snippets,
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js
index 85d8e3caffb2..e84ac357e3fb 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js
@@ -177,7 +177,11 @@ export function Fragment(node, context) {
 	}
 
 	if (has_await) {
-		return b.block([b.stmt(b.call('$.async_body', b.arrow([], b.block(body), true)))]);
+		return b.block([
+			b.stmt(
+				b.call('$.async_body', b.id('$$anchor'), b.arrow([b.id('$$anchor')], b.block(body), true))
+			)
+		]);
 	} else {
 		return b.block(body);
 	}
diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 45c78ff926b9..f284ec1831ed 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -1,5 +1,4 @@
-/** @import { Effect, Value } from '#client' */
-
+/** @import { Effect, TemplateNode, Value } from '#client' */
 import { DESTROYED } from '#client/constants';
 import { DEV } from 'esm-env';
 import { component_context, is_runes, set_component_context } from '../context.js';
@@ -28,6 +27,7 @@ import {
 	set_hydrating,
 	skip_nodes
 } from '../dom/hydration.js';
+import { create_text } from '../dom/operations.js';
 
 /**
  *
@@ -197,9 +197,10 @@ export function unset_context() {
 }
 
 /**
- * @param {() => Promise<void>} fn
+ * @param {TemplateNode} anchor
+ * @param {(target: TemplateNode) => Promise<void>} fn
  */
-export async function async_body(fn) {
+export async function async_body(anchor, fn) {
 	var boundary = get_boundary();
 	var batch = /** @type {Batch} */ (current_batch);
 	var pending = boundary.is_pending();
@@ -217,8 +218,11 @@ export async function async_body(fn) {
 		next_hydrate_node = skip_nodes(false);
 	}
 
+	var target = create_text();
+	anchor.before(target);
+
 	try {
-		var promise = fn();
+		var promise = fn(target);
 	} finally {
 		if (next_hydrate_node) {
 			set_hydrate_node(next_hydrate_node);
@@ -228,6 +232,7 @@ export async function async_body(fn) {
 
 	try {
 		await promise;
+		target.remove();
 	} catch (error) {
 		if (!aborted(active)) {
 			invoke_error_boundary(error, active);

From cc15ffb362ee7445e470477cb786c7787999fc78 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 14:43:06 -0400
Subject: [PATCH 06/26] fix hydration

---
 .../internal/client/dom/blocks/branches.js    | 11 ++++++---
 .../src/internal/client/dom/blocks/if.js      | 24 +++++++------------
 2 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 15d3f0985b11..914f62a9d5aa 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -7,6 +7,7 @@ import {
 	pause_effect,
 	resume_effect
 } from '../../reactivity/effects.js';
+import { hydrate_node, hydrating } from '../hydration.js';
 import { create_text, should_defer_append } from '../operations.js';
 
 /**
@@ -18,7 +19,7 @@ import { create_text, should_defer_append } from '../operations.js';
  */
 export class BranchManager {
 	/** @type {TemplateNode} */
-	#anchor;
+	anchor;
 
 	/** @type {Map<Batch, Key>} */
 	#batches = new Map();
@@ -34,7 +35,7 @@ export class BranchManager {
 	 * @param {TemplateNode} anchor
 	 */
 	constructor(anchor) {
-		this.#anchor = anchor;
+		this.anchor = anchor;
 	}
 
 	#commit = () => {
@@ -58,7 +59,7 @@ export class BranchManager {
 			/** @type {TemplateNode} */ (offscreen.fragment.lastChild).remove();
 
 			// ...and append the fragment
-			this.#anchor.before(offscreen.fragment);
+			this.anchor.before(offscreen.fragment);
 		}
 
 		this.#batches.delete(batch);
@@ -132,6 +133,10 @@ export class BranchManager {
 
 			batch.add_callback(this.#commit);
 		} else {
+			if (hydrating) {
+				this.anchor = hydrate_node;
+			}
+
 			this.#commit();
 		}
 	}
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 23a4555e5bac..0be665fb07e9 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -29,9 +29,6 @@ export function if_block(node, fn, elseif = false) {
 
 	var anchor = node;
 
-	/** @type {typeof UNINITIALIZED | boolean | null} */
-	var condition = UNINITIALIZED;
-
 	var flags = elseif ? EFFECT_TRANSPARENT : 0;
 
 	var has_branch = false;
@@ -44,12 +41,9 @@ export function if_block(node, fn, elseif = false) {
 	var branches = new BranchManager(anchor);
 
 	const update_branch = (
-		/** @type {boolean | null} */ new_condition,
+		/** @type {boolean} */ condition,
 		/** @type {null | ((anchor: Node) => void)} */ fn
 	) => {
-		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
-		let mismatch = false;
-
 		if (hydrating) {
 			const is_else = read_hydration_instruction(anchor) === HYDRATION_START_ELSE;
 
@@ -59,24 +53,24 @@ export function if_block(node, fn, elseif = false) {
 				anchor = skip_nodes();
 
 				set_hydrate_node(anchor);
+				branches.anchor = anchor;
+
 				set_hydrating(false);
-				mismatch = true;
+				branches.ensure(condition, fn ?? noop);
+				set_hydrating(true);
+
+				return;
 			}
 		}
 
-		branches.ensure(new_condition, fn ?? noop);
-
-		if (mismatch) {
-			// continue in hydration mode
-			set_hydrating(true);
-		}
+		branches.ensure(condition, fn ?? noop);
 	};
 
 	block(() => {
 		has_branch = false;
 		fn(set_branch);
 		if (!has_branch) {
-			update_branch(null, null);
+			update_branch(false, null);
 		}
 	}, flags);
 

From 5b17fdd2d05247b78bae951d06773c327ead409d Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 14:50:48 -0400
Subject: [PATCH 07/26] simplify

---
 .../src/internal/client/dom/blocks/if.js      | 51 ++++++++-----------
 1 file changed, 21 insertions(+), 30 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 0be665fb07e9..8190a4325eb6 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -2,7 +2,6 @@
 import { EFFECT_TRANSPARENT } from '#client/constants';
 import {
 	hydrate_next,
-	hydrate_node,
 	hydrating,
 	read_hydration_instruction,
 	skip_nodes,
@@ -10,7 +9,7 @@ import {
 	set_hydrating
 } from '../hydration.js';
 import { block } from '../../reactivity/effects.js';
-import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
+import { HYDRATION_START_ELSE } from '../../../../constants.js';
 import { BranchManager } from './branches.js';
 import { noop } from '../../../shared/utils.js';
 
@@ -27,54 +26,46 @@ export function if_block(node, fn, elseif = false) {
 		hydrate_next();
 	}
 
-	var anchor = node;
-
+	var branches = new BranchManager(node);
 	var flags = elseif ? EFFECT_TRANSPARENT : 0;
 
-	var has_branch = false;
-
-	const set_branch = (/** @type {(anchor: Node) => void} */ fn, flag = true) => {
-		has_branch = true;
-		update_branch(flag, fn);
-	};
-
-	var branches = new BranchManager(anchor);
-
-	const update_branch = (
-		/** @type {boolean} */ condition,
-		/** @type {null | ((anchor: Node) => void)} */ fn
-	) => {
+	/**
+	 * @param {boolean} condition,
+	 * @param {((anchor: Node) => void)} fn
+	 */
+	function update_branch(condition, fn) {
 		if (hydrating) {
-			const is_else = read_hydration_instruction(anchor) === HYDRATION_START_ELSE;
+			const is_else = read_hydration_instruction(node) === HYDRATION_START_ELSE;
 
-			if (!!condition === is_else) {
+			if (condition === is_else) {
 				// Hydration mismatch: remove everything inside the anchor and start fresh.
 				// This could happen with `{#if browser}...{/if}`, for example
-				anchor = skip_nodes();
+				var anchor = skip_nodes();
 
 				set_hydrate_node(anchor);
 				branches.anchor = anchor;
 
 				set_hydrating(false);
-				branches.ensure(condition, fn ?? noop);
+				branches.ensure(condition, fn);
 				set_hydrating(true);
 
 				return;
 			}
 		}
 
-		branches.ensure(condition, fn ?? noop);
-	};
+		branches.ensure(condition, fn);
+	}
 
 	block(() => {
-		has_branch = false;
-		fn(set_branch);
+		var has_branch = false;
+
+		fn((fn, flag = true) => {
+			has_branch = true;
+			update_branch(flag, fn);
+		});
+
 		if (!has_branch) {
-			update_branch(false, null);
+			update_branch(false, noop);
 		}
 	}, flags);
-
-	if (hydrating) {
-		anchor = hydrate_node;
-	}
 }

From ff48b2329c77e329128dcc151c634fcdce973a9a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 15:19:52 -0400
Subject: [PATCH 08/26] all tests passing

---
 .../src/internal/client/dom/blocks/branches.js | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 914f62a9d5aa..b977ba4e667c 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -40,6 +40,10 @@ export class BranchManager {
 
 	#commit = () => {
 		var batch = /** @type {Batch} */ (current_batch);
+
+		// if this batch was made obsolete, bail
+		if (!this.#batches.has(batch)) return;
+
 		var key = /** @type {Key} */ (this.#batches.get(batch));
 
 		var onscreen = this.#onscreen.get(key);
@@ -64,6 +68,20 @@ export class BranchManager {
 
 		this.#batches.delete(batch);
 
+		for (const [b, k] of this.#batches) {
+			if (b === batch) break;
+
+			const offscreen = this.#offscreen.get(k);
+
+			if (offscreen) {
+				destroy_effect(offscreen.effect);
+				this.#offscreen.delete(k);
+			}
+
+			this.#batches.delete(b);
+		}
+
+		// outro/destroy effects
 		for (const [k, effect] of this.#onscreen) {
 			if (k === key) continue;
 

From d168bb137ea82e129bf553a8752f062f56adfce6 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 15:34:44 -0400
Subject: [PATCH 09/26] key blocks

---
 .../internal/client/dom/blocks/branches.js    |  8 +++
 .../src/internal/client/dom/blocks/key.js     | 68 ++-----------------
 2 files changed, 14 insertions(+), 62 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index b977ba4e667c..16ca4b1d6827 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -1,4 +1,5 @@
 /** @import { Effect, TemplateNode } from '#client' */
+import { is_runes } from '../../context.js';
 import { Batch, current_batch } from '../../reactivity/batch.js';
 import {
 	branch,
@@ -30,6 +31,8 @@ export class BranchManager {
 	/** @type {Map<Key, Branch>} */
 	#offscreen = new Map();
 
+	#legacy = !is_runes();
+
 	/**
 	 *
 	 * @param {TemplateNode} anchor
@@ -117,6 +120,11 @@ export class BranchManager {
 	ensure(key, fn) {
 		var batch = /** @type {Batch} */ (current_batch);
 
+		// key blocks in Svelte <5 had stupid semantics
+		if (this.#legacy && typeof key === 'object') {
+			key = {};
+		}
+
 		if (!this.#onscreen.has(key) && !this.#offscreen.has(key)) {
 			var fragment = document.createDocumentFragment();
 			var target = create_text();
diff --git a/packages/svelte/src/internal/client/dom/blocks/key.js b/packages/svelte/src/internal/client/dom/blocks/key.js
index 5e3c42019f33..143b225d7f4d 100644
--- a/packages/svelte/src/internal/client/dom/blocks/key.js
+++ b/packages/svelte/src/internal/client/dom/blocks/key.js
@@ -1,12 +1,7 @@
-/** @import { Effect, TemplateNode } from '#client' */
-/** @import { Batch } from '../../reactivity/batch.js'; */
-import { UNINITIALIZED } from '../../../../constants.js';
-import { block, branch, pause_effect } from '../../reactivity/effects.js';
-import { not_equal, safe_not_equal } from '../../reactivity/equality.js';
-import { is_runes } from '../../context.js';
-import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
-import { create_text, should_defer_append } from '../operations.js';
-import { current_batch } from '../../reactivity/batch.js';
+/** @import { TemplateNode } from '#client' */
+import { block } from '../../reactivity/effects.js';
+import { hydrate_next, hydrating } from '../hydration.js';
+import { BranchManager } from './branches.js';
 
 /**
  * @template V
@@ -20,60 +15,9 @@ export function key(node, get_key, render_fn) {
 		hydrate_next();
 	}
 
-	var anchor = node;
-
-	/** @type {V | typeof UNINITIALIZED} */
-	var key = UNINITIALIZED;
-
-	/** @type {Effect} */
-	var effect;
-
-	/** @type {Effect} */
-	var pending_effect;
-
-	/** @type {DocumentFragment | null} */
-	var offscreen_fragment = null;
-
-	var changed = is_runes() ? not_equal : safe_not_equal;
-
-	function commit() {
-		if (effect) {
-			pause_effect(effect);
-		}
-
-		if (offscreen_fragment !== null) {
-			// remove the anchor
-			/** @type {Text} */ (offscreen_fragment.lastChild).remove();
-
-			anchor.before(offscreen_fragment);
-			offscreen_fragment = null;
-		}
-
-		effect = pending_effect;
-	}
+	var branches = new BranchManager(node);
 
 	block(() => {
-		if (changed(key, (key = get_key()))) {
-			var target = anchor;
-
-			var defer = should_defer_append();
-
-			if (defer) {
-				offscreen_fragment = document.createDocumentFragment();
-				offscreen_fragment.append((target = create_text()));
-			}
-
-			pending_effect = branch(() => render_fn(target));
-
-			if (defer) {
-				/** @type {Batch} */ (current_batch).add_callback(commit);
-			} else {
-				commit();
-			}
-		}
+		branches.ensure(get_key(), render_fn);
 	});
-
-	if (hydrating) {
-		anchor = hydrate_node;
-	}
 }

From e3b15104ec9008b78a8afe19d19de055eeef5631 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 15:41:01 -0400
Subject: [PATCH 10/26] snippets

---
 .../src/internal/client/dom/blocks/snippet.js | 30 +++++--------------
 1 file changed, 7 insertions(+), 23 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/snippet.js b/packages/svelte/src/internal/client/dom/blocks/snippet.js
index 32d88d4c606a..6c0fd71789f1 100644
--- a/packages/svelte/src/internal/client/dom/blocks/snippet.js
+++ b/packages/svelte/src/internal/client/dom/blocks/snippet.js
@@ -1,8 +1,8 @@
 /** @import { Snippet } from 'svelte' */
-/** @import { Effect, TemplateNode } from '#client' */
+/** @import { TemplateNode } from '#client' */
 /** @import { Getters } from '#shared' */
 import { EFFECT_TRANSPARENT, ELEMENT_NODE } from '#client/constants';
-import { branch, block, destroy_effect, teardown } from '../../reactivity/effects.js';
+import { block, teardown } from '../../reactivity/effects.js';
 import {
 	dev_current_component_function,
 	set_dev_current_component_function
@@ -14,8 +14,8 @@ import * as w from '../../warnings.js';
 import * as e from '../../errors.js';
 import { DEV } from 'esm-env';
 import { get_first_child, get_next_sibling } from '../operations.js';
-import { noop } from '../../../shared/utils.js';
 import { prevent_snippet_stringification } from '../../../shared/validate.js';
+import { BranchManager } from './branches.js';
 
 /**
  * @template {(node: TemplateNode, ...args: any[]) => void} SnippetFn
@@ -25,33 +25,17 @@ import { prevent_snippet_stringification } from '../../../shared/validate.js';
  * @returns {void}
  */
 export function snippet(node, get_snippet, ...args) {
-	var anchor = node;
-
-	/** @type {SnippetFn | null | undefined} */
-	// @ts-ignore
-	var snippet = noop;
-
-	/** @type {Effect | null} */
-	var snippet_effect;
+	var branches = new BranchManager(node);
 
 	block(() => {
-		if (snippet === (snippet = get_snippet())) return;
+		const snippet = get_snippet();
 
-		if (snippet_effect) {
-			destroy_effect(snippet_effect);
-			snippet_effect = null;
-		}
-
-		if (DEV && snippet == null) {
+		if (snippet == null) {
 			e.invalid_snippet();
 		}
 
-		snippet_effect = branch(() => /** @type {SnippetFn} */ (snippet)(anchor, ...args));
+		branches.ensure(snippet, (anchor) => snippet(anchor, ...args));
 	}, EFFECT_TRANSPARENT);
-
-	if (hydrating) {
-		anchor = hydrate_node;
-	}
 }
 
 /**

From 3c2eb4c15df4823952a6d48cf053b89a4d6e89c3 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 16:16:07 -0400
Subject: [PATCH 11/26] fix

---
 .../internal/client/dom/blocks/branches.js    | 19 +++---
 .../src/internal/client/dom/blocks/if.js      |  4 +-
 .../src/internal/client/dom/blocks/snippet.js |  6 +-
 .../client/dom/blocks/svelte-component.js     | 62 ++-----------------
 4 files changed, 20 insertions(+), 71 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 16ca4b1d6827..358b87ee41c7 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -57,16 +57,17 @@ export class BranchManager {
 		} else {
 			// effect is currently offscreen. put it in the DOM
 			var offscreen = this.#offscreen.get(key);
-			if (!offscreen) throw new Error('This should never happen!');
 
-			this.#onscreen.set(key, offscreen.effect);
-			this.#offscreen.delete(key);
+			if (offscreen) {
+				this.#onscreen.set(key, offscreen.effect);
+				this.#offscreen.delete(key);
 
-			// remove the anchor...
-			/** @type {TemplateNode} */ (offscreen.fragment.lastChild).remove();
+				// remove the anchor...
+				/** @type {TemplateNode} */ (offscreen.fragment.lastChild).remove();
 
-			// ...and append the fragment
-			this.anchor.before(offscreen.fragment);
+				// ...and append the fragment
+				this.anchor.before(offscreen.fragment);
+			}
 		}
 
 		this.#batches.delete(batch);
@@ -115,7 +116,7 @@ export class BranchManager {
 	/**
 	 *
 	 * @param {any} key
-	 * @param {(target: TemplateNode) => void} fn
+	 * @param {null | ((target: TemplateNode) => void)} fn
 	 */
 	ensure(key, fn) {
 		var batch = /** @type {Batch} */ (current_batch);
@@ -125,7 +126,7 @@ export class BranchManager {
 			key = {};
 		}
 
-		if (!this.#onscreen.has(key) && !this.#offscreen.has(key)) {
+		if (fn && !this.#onscreen.has(key) && !this.#offscreen.has(key)) {
 			var fragment = document.createDocumentFragment();
 			var target = create_text();
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 8190a4325eb6..1d6b2d9163a6 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -31,7 +31,7 @@ export function if_block(node, fn, elseif = false) {
 
 	/**
 	 * @param {boolean} condition,
-	 * @param {((anchor: Node) => void)} fn
+	 * @param {null | ((anchor: Node) => void)} fn
 	 */
 	function update_branch(condition, fn) {
 		if (hydrating) {
@@ -65,7 +65,7 @@ export function if_block(node, fn, elseif = false) {
 		});
 
 		if (!has_branch) {
-			update_branch(false, noop);
+			update_branch(false, null);
 		}
 	}, flags);
 }
diff --git a/packages/svelte/src/internal/client/dom/blocks/snippet.js b/packages/svelte/src/internal/client/dom/blocks/snippet.js
index 6c0fd71789f1..0c4948aca027 100644
--- a/packages/svelte/src/internal/client/dom/blocks/snippet.js
+++ b/packages/svelte/src/internal/client/dom/blocks/snippet.js
@@ -28,13 +28,13 @@ export function snippet(node, get_snippet, ...args) {
 	var branches = new BranchManager(node);
 
 	block(() => {
-		const snippet = get_snippet();
+		const snippet = get_snippet() ?? null;
 
-		if (snippet == null) {
+		if (DEV && snippet == null) {
 			e.invalid_snippet();
 		}
 
-		branches.ensure(snippet, (anchor) => snippet(anchor, ...args));
+		branches.ensure(snippet, snippet && ((anchor) => snippet(anchor, ...args)));
 	}, EFFECT_TRANSPARENT);
 }
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/svelte-component.js b/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
index 2697722b3953..53821a95ca39 100644
--- a/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
+++ b/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
@@ -5,6 +5,8 @@ import { block, branch, pause_effect } from '../../reactivity/effects.js';
 import { current_batch } from '../../reactivity/batch.js';
 import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
 import { create_text, should_defer_append } from '../operations.js';
+import { BranchManager } from './branches.js';
+import { noop } from '../../../shared/utils.js';
 
 /**
  * @template P
@@ -19,64 +21,10 @@ export function component(node, get_component, render_fn) {
 		hydrate_next();
 	}
 
-	var anchor = node;
-
-	/** @type {C} */
-	var component;
-
-	/** @type {Effect | null} */
-	var effect;
-
-	/** @type {DocumentFragment | null} */
-	var offscreen_fragment = null;
-
-	/** @type {Effect | null} */
-	var pending_effect = null;
-
-	function commit() {
-		if (effect) {
-			pause_effect(effect);
-			effect = null;
-		}
-
-		if (offscreen_fragment) {
-			// remove the anchor
-			/** @type {Text} */ (offscreen_fragment.lastChild).remove();
-
-			anchor.before(offscreen_fragment);
-			offscreen_fragment = null;
-		}
-
-		effect = pending_effect;
-		pending_effect = null;
-	}
+	var branches = new BranchManager(node);
 
 	block(() => {
-		if (component === (component = get_component())) return;
-
-		var defer = should_defer_append();
-
-		if (component) {
-			var target = anchor;
-
-			if (defer) {
-				offscreen_fragment = document.createDocumentFragment();
-				offscreen_fragment.append((target = create_text()));
-				if (effect) {
-					/** @type {Batch} */ (current_batch).skipped_effects.add(effect);
-				}
-			}
-			pending_effect = branch(() => render_fn(target, component));
-		}
-
-		if (defer) {
-			/** @type {Batch} */ (current_batch).add_callback(commit);
-		} else {
-			commit();
-		}
+		var component = get_component() ?? null;
+		branches.ensure(component, component && ((target) => render_fn(target, component)));
 	}, EFFECT_TRANSPARENT);
-
-	if (hydrating) {
-		anchor = hydrate_node;
-	}
 }

From 5f01369c68d7c68d6225ee997387cedc6a15aa07 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 16:17:29 -0400
Subject: [PATCH 12/26] tidy up

---
 packages/svelte/src/internal/client/dom/blocks/if.js   |  1 -
 .../src/internal/client/dom/blocks/svelte-component.js | 10 +++-------
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/if.js b/packages/svelte/src/internal/client/dom/blocks/if.js
index 1d6b2d9163a6..7fa5ca464dd1 100644
--- a/packages/svelte/src/internal/client/dom/blocks/if.js
+++ b/packages/svelte/src/internal/client/dom/blocks/if.js
@@ -11,7 +11,6 @@ import {
 import { block } from '../../reactivity/effects.js';
 import { HYDRATION_START_ELSE } from '../../../../constants.js';
 import { BranchManager } from './branches.js';
-import { noop } from '../../../shared/utils.js';
 
 // TODO reinstate https://github.com/sveltejs/svelte/pull/15250
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/svelte-component.js b/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
index 53821a95ca39..134e57e62710 100644
--- a/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
+++ b/packages/svelte/src/internal/client/dom/blocks/svelte-component.js
@@ -1,12 +1,8 @@
-/** @import { TemplateNode, Dom, Effect } from '#client' */
-/** @import { Batch } from '../../reactivity/batch.js'; */
+/** @import { TemplateNode, Dom } from '#client' */
 import { EFFECT_TRANSPARENT } from '#client/constants';
-import { block, branch, pause_effect } from '../../reactivity/effects.js';
-import { current_batch } from '../../reactivity/batch.js';
-import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
-import { create_text, should_defer_append } from '../operations.js';
+import { block } from '../../reactivity/effects.js';
+import { hydrate_next, hydrating } from '../hydration.js';
 import { BranchManager } from './branches.js';
-import { noop } from '../../../shared/utils.js';
 
 /**
  * @template P

From 3a1820d102d4146651aefe82fb7eb4ce51e27ef8 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 17:38:28 -0400
Subject: [PATCH 13/26] WIP await

---
 .../src/internal/client/dom/blocks/await.js   | 173 ++++++------------
 .../src/internal/client/reactivity/async.js   |   2 +-
 2 files changed, 59 insertions(+), 116 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index e7917fbd9e45..d71c8dcc3d49 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -23,6 +23,8 @@ import {
 	set_dev_stack
 } from '../../context.js';
 import { flushSync, is_flushing_sync } from '../../reactivity/batch.js';
+import { BranchManager } from './branches.js';
+import { capture, unset_context } from '../../reactivity/async.js';
 
 const PENDING = 0;
 const THEN = 1;
@@ -33,7 +35,7 @@ const CATCH = 2;
 /**
  * @template V
  * @param {TemplateNode} node
- * @param {(() => Promise<V>)} get_input
+ * @param {(() => any)} get_input
  * @param {null | ((anchor: Node) => void)} pending_fn
  * @param {null | ((anchor: Node, value: Source<V>) => void)} then_fn
  * @param {null | ((anchor: Node, error: unknown) => void)} catch_fn
@@ -44,149 +46,95 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 		hydrate_next();
 	}
 
-	var anchor = node;
 	var runes = is_runes();
-	var active_component_context = component_context;
-
-	/** @type {any} */
-	var component_function = DEV ? component_context?.function : null;
-	var dev_original_stack = DEV ? dev_stack : null;
-
-	/** @type {V | Promise<V> | typeof UNINITIALIZED} */
-	var input = UNINITIALIZED;
-
-	/** @type {Effect | null} */
-	var pending_effect;
-
-	/** @type {Effect | null} */
-	var then_effect;
-
-	/** @type {Effect | null} */
-	var catch_effect;
 
 	var input_source = runes
 		? source(/** @type {V} */ (undefined))
 		: mutable_source(/** @type {V} */ (undefined), false, false);
 	var error_source = runes ? source(undefined) : mutable_source(undefined, false, false);
-	var resolved = false;
-	/**
-	 * @param {AwaitState} state
-	 * @param {boolean} restore
-	 */
-	function update(state, restore) {
-		resolved = true;
-
-		if (restore) {
-			set_active_effect(effect);
-			set_active_reaction(effect); // TODO do we need both?
-			set_component_context(active_component_context);
-			if (DEV) {
-				set_dev_current_component_function(component_function);
-				set_dev_stack(dev_original_stack);
-			}
-		}
 
-		try {
-			if (state === PENDING && pending_fn) {
-				if (pending_effect) resume_effect(pending_effect);
-				else pending_effect = branch(() => pending_fn(anchor));
-			}
-
-			if (state === THEN && then_fn) {
-				if (then_effect) resume_effect(then_effect);
-				else then_effect = branch(() => then_fn(anchor, input_source));
-			}
-
-			if (state === CATCH && catch_fn) {
-				if (catch_effect) resume_effect(catch_effect);
-				else catch_effect = branch(() => catch_fn(anchor, error_source));
-			}
-
-			if (state !== PENDING && pending_effect) {
-				pause_effect(pending_effect, () => (pending_effect = null));
-			}
-
-			if (state !== THEN && then_effect) {
-				pause_effect(then_effect, () => (then_effect = null));
-			}
-
-			if (state !== CATCH && catch_effect) {
-				pause_effect(catch_effect, () => (catch_effect = null));
-			}
-		} finally {
-			if (restore) {
-				if (DEV) {
-					set_dev_current_component_function(null);
-					set_dev_stack(null);
-				}
+	var branches = new BranchManager(node);
 
-				set_component_context(null);
-				set_active_reaction(null);
-				set_active_effect(null);
-
-				// without this, the DOM does not update until two ticks after the promise
-				// resolves, which is unexpected behaviour (and somewhat irksome to test)
-				if (!is_flushing_sync) flushSync();
-			}
-		}
-	}
-
-	var effect = block(() => {
-		if (input === (input = get_input())) return;
+	block(() => {
+		var input = get_input();
+		var destroyed = false;
 
 		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
 		// @ts-ignore coercing `anchor` to a `Comment` causes TypeScript and Prettier to fight
-		let mismatch = hydrating && is_promise(input) === (anchor.data === HYDRATION_START_ELSE);
+		let mismatch = hydrating && is_promise(input) === (node.data === HYDRATION_START_ELSE);
 
 		if (mismatch) {
 			// Hydration mismatch: remove everything inside the anchor and start fresh
-			anchor = skip_nodes();
-
-			set_hydrate_node(anchor);
+			set_hydrate_node(skip_nodes());
 			set_hydrating(false);
 			mismatch = true;
 		}
 
 		if (is_promise(input)) {
-			var promise = input;
+			var restore = capture();
+			var resolved = false;
+
+			/**
+			 * @param {() => void} fn
+			 */
+			const resolve = (fn) => {
+				if (destroyed) return;
+
+				resolved = true;
+				restore();
+
+				if (hydrating) {
+					// we want to restore everything _except_ this
+					set_hydrating(false);
+				}
+
+				try {
+					fn();
+				} finally {
+					unset_context();
 
-			resolved = false;
+					// without this, the DOM does not update until two ticks after the promise
+					// resolves, which is unexpected behaviour (and somewhat irksome to test)
+					if (!is_flushing_sync) flushSync();
+				}
+			};
 
-			promise.then(
+			input.then(
 				(value) => {
-					if (promise !== input) return;
-					// we technically could use `set` here since it's on the next microtick
-					// but let's use internal_set for consistency and just to be safe
-					internal_set(input_source, value);
-					update(THEN, true);
+					resolve(() => {
+						internal_set(input_source, value);
+						branches.ensure(THEN, then_fn && ((target) => then_fn(target, input_source)));
+					});
 				},
 				(error) => {
-					if (promise !== input) return;
-					// we technically could use `set` here since it's on the next microtick
-					// but let's use internal_set for consistency and just to be safe
-					internal_set(error_source, error);
-					update(CATCH, true);
-					if (!catch_fn) {
-						// Rethrow the error if no catch block exists
-						throw error_source.v;
-					}
+					resolve(() => {
+						internal_set(error_source, error);
+						branches.ensure(THEN, catch_fn && ((target) => catch_fn(target, error_source)));
+
+						if (!catch_fn) {
+							// Rethrow the error if no catch block exists
+							throw error_source.v;
+						}
+					});
 				}
 			);
 
 			if (hydrating) {
-				if (pending_fn) {
-					pending_effect = branch(() => pending_fn(anchor));
-				}
+				branches.ensure(PENDING, pending_fn);
 			} else {
 				// Wait a microtask before checking if we should show the pending state as
-				// the promise might have resolved by the next microtask.
+				// the promise might have resolved by then
 				queue_micro_task(() => {
-					if (!resolved) update(PENDING, true);
+					if (!resolved) {
+						resolve(() => {
+							branches.ensure(PENDING, pending_fn);
+						});
+					}
 				});
 			}
 		} else {
 			internal_set(input_source, input);
-			update(THEN, false);
+			branches.ensure(THEN, then_fn && ((target) => then_fn(target, input_source)));
 		}
 
 		if (mismatch) {
@@ -194,11 +142,6 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			set_hydrating(true);
 		}
 
-		// Set the input to something else, in order to disable the promise callbacks
-		return () => (input = UNINITIALIZED);
+		return () => (destroyed = true);
 	});
-
-	if (hydrating) {
-		anchor = hydrate_node;
-	}
 }
diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index f284ec1831ed..45369900b21a 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -80,7 +80,7 @@ export function flatten(sync, async, fn) {
  * some asynchronous work has happened (so that e.g. `await a + b`
  * causes `b` to be registered as a dependency).
  */
-function capture() {
+export function capture() {
 	var previous_effect = active_effect;
 	var previous_reaction = active_reaction;
 	var previous_component_context = component_context;

From 82fe9449ac273e4ee569a717d6bcc82a3212e84e Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 19:21:26 -0400
Subject: [PATCH 14/26] tidy up

---
 .../src/internal/client/dom/blocks/await.js   | 22 ++++++-------------
 .../samples/await-pending-destroy/_config.js  |  3 ++-
 2 files changed, 9 insertions(+), 16 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index d71c8dcc3d49..dd617168fb61 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -1,27 +1,17 @@
-/** @import { Effect, Source, TemplateNode } from '#client' */
-import { DEV } from 'esm-env';
+/** @import { Source, TemplateNode } from '#client' */
 import { is_promise } from '../../../shared/utils.js';
-import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
+import { block } from '../../reactivity/effects.js';
 import { internal_set, mutable_source, source } from '../../reactivity/sources.js';
-import { set_active_effect, set_active_reaction } from '../../runtime.js';
 import {
 	hydrate_next,
-	hydrate_node,
 	hydrating,
 	skip_nodes,
 	set_hydrate_node,
 	set_hydrating
 } from '../hydration.js';
 import { queue_micro_task } from '../task.js';
-import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
-import {
-	component_context,
-	dev_stack,
-	is_runes,
-	set_component_context,
-	set_dev_current_component_function,
-	set_dev_stack
-} from '../../context.js';
+import { HYDRATION_START_ELSE } from '../../../../constants.js';
+import { is_runes } from '../../context.js';
 import { flushSync, is_flushing_sync } from '../../reactivity/batch.js';
 import { BranchManager } from './branches.js';
 import { capture, unset_context } from '../../reactivity/async.js';
@@ -142,6 +132,8 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			set_hydrating(true);
 		}
 
-		return () => (destroyed = true);
+		return () => {
+			destroyed = true;
+		};
 	});
 }
diff --git a/packages/svelte/tests/runtime-runes/samples/await-pending-destroy/_config.js b/packages/svelte/tests/runtime-runes/samples/await-pending-destroy/_config.js
index 1725cd8f6fb0..9ef598de6c83 100644
--- a/packages/svelte/tests/runtime-runes/samples/await-pending-destroy/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/await-pending-destroy/_config.js
@@ -1,3 +1,4 @@
+import { tick } from 'svelte';
 import { test } from '../../test';
 
 /**
@@ -77,7 +78,7 @@ export default test({
 			const { promise, reject } = promiseWithResolver();
 			component.promise = promise;
 			// wait for rendering
-			await Promise.resolve();
+			await tick();
 
 			// remove the promise
 			component.promise = null;

From fe1cc8f2e31af4c3ad7d42f691e75ac6644fea69 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 19:34:45 -0400
Subject: [PATCH 15/26] fix

---
 .../src/internal/client/reactivity/async.js   | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 45369900b21a..76ade0950462 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -1,7 +1,13 @@
 /** @import { Effect, TemplateNode, Value } from '#client' */
 import { DESTROYED } from '#client/constants';
 import { DEV } from 'esm-env';
-import { component_context, is_runes, set_component_context } from '../context.js';
+import {
+	component_context,
+	dev_stack,
+	is_runes,
+	set_component_context,
+	set_dev_stack
+} from '../context.js';
 import { get_boundary } from '../dom/blocks/boundary.js';
 import { invoke_error_boundary } from '../error-handling.js';
 import {
@@ -92,6 +98,10 @@ export function capture() {
 		var previous_hydrate_node = hydrate_node;
 	}
 
+	if (DEV) {
+		var previous_dev_stack = dev_stack;
+	}
+
 	return function restore() {
 		set_active_effect(previous_effect);
 		set_active_reaction(previous_reaction);
@@ -105,6 +115,7 @@ export function capture() {
 
 		if (DEV) {
 			set_from_async_derived(null);
+			set_dev_stack(previous_dev_stack);
 		}
 	};
 }
@@ -193,7 +204,11 @@ export function unset_context() {
 	set_active_effect(null);
 	set_active_reaction(null);
 	set_component_context(null);
-	if (DEV) set_from_async_derived(null);
+
+	if (DEV) {
+		set_from_async_derived(null);
+		set_dev_stack(null);
+	}
 }
 
 /**

From cb7203a572b9c5e9a9236395e4f43756121a763e Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 19:45:11 -0400
Subject: [PATCH 16/26] neaten up

---
 .../src/internal/client/dom/blocks/await.js   | 27 +++++++++----------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index dd617168fb61..cab6bdc147b5 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -10,7 +10,7 @@ import {
 	set_hydrating
 } from '../hydration.js';
 import { queue_micro_task } from '../task.js';
-import { HYDRATION_START_ELSE } from '../../../../constants.js';
+import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
 import { is_runes } from '../../context.js';
 import { flushSync, is_flushing_sync } from '../../reactivity/batch.js';
 import { BranchManager } from './branches.js';
@@ -38,10 +38,9 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 
 	var runes = is_runes();
 
-	var input_source = runes
-		? source(/** @type {V} */ (undefined))
-		: mutable_source(/** @type {V} */ (undefined), false, false);
-	var error_source = runes ? source(undefined) : mutable_source(undefined, false, false);
+	var v = /** @type {V} */ (UNINITIALIZED);
+	var value = runes ? source(v) : mutable_source(v, false, false);
+	var error = runes ? source(v) : mutable_source(v, false, false);
 
 	var branches = new BranchManager(node);
 
@@ -90,20 +89,20 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			};
 
 			input.then(
-				(value) => {
+				(v) => {
 					resolve(() => {
-						internal_set(input_source, value);
-						branches.ensure(THEN, then_fn && ((target) => then_fn(target, input_source)));
+						internal_set(value, v);
+						branches.ensure(THEN, then_fn && ((target) => then_fn(target, value)));
 					});
 				},
-				(error) => {
+				(e) => {
 					resolve(() => {
-						internal_set(error_source, error);
-						branches.ensure(THEN, catch_fn && ((target) => catch_fn(target, error_source)));
+						internal_set(error, e);
+						branches.ensure(THEN, catch_fn && ((target) => catch_fn(target, error)));
 
 						if (!catch_fn) {
 							// Rethrow the error if no catch block exists
-							throw error_source.v;
+							throw error.v;
 						}
 					});
 				}
@@ -123,8 +122,8 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 				});
 			}
 		} else {
-			internal_set(input_source, input);
-			branches.ensure(THEN, then_fn && ((target) => then_fn(target, input_source)));
+			internal_set(value, input);
+			branches.ensure(THEN, then_fn && ((target) => then_fn(target, value)));
 		}
 
 		if (mismatch) {

From 82a63f278e3d970939f74fec02f7b7c7af3852ca Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 19:46:21 -0400
Subject: [PATCH 17/26] unused

---
 packages/svelte/src/internal/client/dom/blocks/await.js | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index cab6bdc147b5..53d3ba67f9ec 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -56,7 +56,6 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			// Hydration mismatch: remove everything inside the anchor and start fresh
 			set_hydrate_node(skip_nodes());
 			set_hydrating(false);
-			mismatch = true;
 		}
 
 		if (is_promise(input)) {

From 446ffc252bea2eee85ee76ffbd8a485e7f5a66d3 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 19:47:31 -0400
Subject: [PATCH 18/26] tweak

---
 packages/svelte/src/internal/client/dom/blocks/await.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index 53d3ba67f9ec..7fe04934bc60 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -49,7 +49,7 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 		var destroyed = false;
 
 		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
-		// @ts-ignore coercing `anchor` to a `Comment` causes TypeScript and Prettier to fight
+		// @ts-ignore coercing `node` to a `Comment` causes TypeScript and Prettier to fight
 		let mismatch = hydrating && is_promise(input) === (node.data === HYDRATION_START_ELSE);
 
 		if (mismatch) {

From a38230421977648b1b6f911ad051587d5e807ac0 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 20:54:37 -0400
Subject: [PATCH 19/26] elements

---
 .../internal/client/dom/blocks/branches.js    | 80 ++++++++++++-------
 .../client/dom/blocks/svelte-element.js       | 79 ++++++++----------
 packages/svelte/src/internal/client/render.js |  1 +
 3 files changed, 84 insertions(+), 76 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 358b87ee41c7..338c70a13ec8 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -8,6 +8,7 @@ import {
 	pause_effect,
 	resume_effect
 } from '../../reactivity/effects.js';
+import { set_should_intro, should_intro } from '../../render.js';
 import { hydrate_node, hydrating } from '../hydration.js';
 import { create_text, should_defer_append } from '../operations.js';
 
@@ -34,11 +35,18 @@ export class BranchManager {
 	#legacy = !is_runes();
 
 	/**
-	 *
+	 * Whether to pause (i.e. outro) on change, or destroy immediately.
+	 * This is necessary for `<svelte:element>`
+	 */
+	#transition = true;
+
+	/**
 	 * @param {TemplateNode} anchor
+	 * @param {boolean} transition
 	 */
-	constructor(anchor) {
+	constructor(anchor, transition = true) {
 		this.anchor = anchor;
+		this.#transition = transition;
 	}
 
 	#commit = () => {
@@ -67,6 +75,7 @@ export class BranchManager {
 
 				// ...and append the fragment
 				this.anchor.before(offscreen.fragment);
+				onscreen = offscreen.effect;
 			}
 		}
 
@@ -89,27 +98,29 @@ export class BranchManager {
 		for (const [k, effect] of this.#onscreen) {
 			if (k === key) continue;
 
-			pause_effect(
-				effect,
-				() => {
-					const keys = Array.from(this.#batches.values());
+			const on_destroy = () => {
+				const keys = Array.from(this.#batches.values());
 
-					if (keys.includes(k)) {
-						// keep the effect offscreen, as another batch will need it
-						var fragment = document.createDocumentFragment();
-						move_effect(effect, fragment);
+				if (keys.includes(k)) {
+					// keep the effect offscreen, as another batch will need it
+					var fragment = document.createDocumentFragment();
+					move_effect(effect, fragment);
 
-						fragment.append(create_text()); // TODO can we avoid this?
+					fragment.append(create_text()); // TODO can we avoid this?
 
-						this.#offscreen.set(k, { effect, fragment });
-					} else {
-						destroy_effect(effect);
-					}
+					this.#offscreen.set(k, { effect, fragment });
+				} else {
+					destroy_effect(effect);
+				}
 
-					this.#onscreen.delete(k);
-				},
-				false
-			);
+				this.#onscreen.delete(k);
+			};
+
+			if (this.#transition || !onscreen) {
+				pause_effect(effect, on_destroy, false);
+			} else {
+				on_destroy();
+			}
 		}
 	};
 
@@ -120,28 +131,35 @@ export class BranchManager {
 	 */
 	ensure(key, fn) {
 		var batch = /** @type {Batch} */ (current_batch);
+		var defer = should_defer_append();
 
 		// key blocks in Svelte <5 had stupid semantics
-		if (this.#legacy && typeof key === 'object') {
+		if (this.#legacy && key !== null && typeof key === 'object') {
 			key = {};
 		}
 
 		if (fn && !this.#onscreen.has(key) && !this.#offscreen.has(key)) {
-			var fragment = document.createDocumentFragment();
-			var target = create_text();
-
-			fragment.append(target);
-
-			this.#offscreen.set(key, {
-				effect: branch(() => fn(target)),
-				fragment
-			});
+			if (defer) {
+				var fragment = document.createDocumentFragment();
+				var target = create_text();
+
+				fragment.append(target);
+
+				this.#offscreen.set(key, {
+					effect: branch(() => fn(target)),
+					fragment
+				});
+			} else {
+				this.#onscreen.set(
+					key,
+					branch(() => fn(this.anchor))
+				);
+			}
 		}
 
 		this.#batches.set(batch, key);
 
-		// TODO in the no-defer case, we could skip the offscreen step
-		if (should_defer_append()) {
+		if (defer) {
 			for (const [k, effect] of this.#onscreen) {
 				if (k === key) {
 					batch.skipped_effects.delete(effect);
diff --git a/packages/svelte/src/internal/client/dom/blocks/svelte-element.js b/packages/svelte/src/internal/client/dom/blocks/svelte-element.js
index 231a3621b1af..6533ff8921a2 100644
--- a/packages/svelte/src/internal/client/dom/blocks/svelte-element.js
+++ b/packages/svelte/src/internal/client/dom/blocks/svelte-element.js
@@ -8,13 +8,7 @@ import {
 	set_hydrating
 } from '../hydration.js';
 import { create_text, get_first_child } from '../operations.js';
-import {
-	block,
-	branch,
-	destroy_effect,
-	pause_effect,
-	resume_effect
-} from '../../reactivity/effects.js';
+import { block, teardown } from '../../reactivity/effects.js';
 import { set_should_intro } from '../../render.js';
 import { current_each_item, set_current_each_item } from './each.js';
 import { active_effect } from '../../runtime.js';
@@ -23,6 +17,7 @@ import { DEV } from 'esm-env';
 import { EFFECT_TRANSPARENT, ELEMENT_NODE } from '#client/constants';
 import { assign_nodes } from '../template.js';
 import { is_raw_text_element } from '../../../../utils.js';
+import { BranchManager } from './branches.js';
 
 /**
  * @param {Comment | Element} node
@@ -42,12 +37,6 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 
 	var filename = DEV && location && component_context?.function[FILENAME];
 
-	/** @type {string | null} */
-	var tag;
-
-	/** @type {string | null} */
-	var current_tag;
-
 	/** @type {null | Element} */
 	var element = null;
 
@@ -58,9 +47,6 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 
 	var anchor = /** @type {TemplateNode} */ (hydrating ? hydrate_node : node);
 
-	/** @type {Effect | null} */
-	var effect;
-
 	/**
 	 * The keyed `{#each ...}` item block, if any, that this element is inside.
 	 * We track this so we can set it when changing the element, allowing any
@@ -68,36 +54,24 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 	 */
 	var each_item_block = current_each_item;
 
+	var branches = new BranchManager(anchor, false);
+
 	block(() => {
 		const next_tag = get_tag() || null;
 		var ns = get_namespace ? get_namespace() : is_svg || next_tag === 'svg' ? NAMESPACE_SVG : null;
 
-		// Assumption: Noone changes the namespace but not the tag (what would that even mean?)
-		if (next_tag === tag) return;
-
-		// See explanation of `each_item_block` above
-		var previous_each_item = current_each_item;
-		set_current_each_item(each_item_block);
-
-		if (effect) {
-			if (next_tag === null) {
-				// start outro
-				pause_effect(effect, () => {
-					effect = null;
-					current_tag = null;
-				});
-			} else if (next_tag === current_tag) {
-				// same tag as is currently rendered — abort outro
-				resume_effect(effect);
-			} else {
-				// tag is changing — destroy immediately, render contents without intro transitions
-				destroy_effect(effect);
-				set_should_intro(false);
-			}
+		if (next_tag === null) {
+			branches.ensure(null, null);
+			set_should_intro(true);
+			return;
 		}
 
-		if (next_tag && next_tag !== current_tag) {
-			effect = branch(() => {
+		branches.ensure(next_tag, (anchor) => {
+			// See explanation of `each_item_block` above
+			var previous_each_item = current_each_item;
+			set_current_each_item(each_item_block);
+
+			if (next_tag) {
 				element = hydrating
 					? /** @type {Element} */ (element)
 					: ns
@@ -149,16 +123,31 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 				/** @type {Effect} */ (active_effect).nodes_end = element;
 
 				anchor.before(element);
-			});
-		}
+			}
+
+			set_current_each_item(previous_each_item);
+
+			if (hydrating) {
+				set_hydrate_node(anchor);
+			}
+		});
 
-		tag = next_tag;
-		if (tag) current_tag = tag;
+		// revert to the default state after the effect has been created
 		set_should_intro(true);
 
-		set_current_each_item(previous_each_item);
+		return () => {
+			if (next_tag) {
+				// if we're in this callback because we're re-running the effect,
+				// disable intros (unless no element is currently displayed)
+				set_should_intro(false);
+			}
+		};
 	}, EFFECT_TRANSPARENT);
 
+	teardown(() => {
+		set_should_intro(true);
+	});
+
 	if (was_hydrating) {
 		set_hydrating(true);
 		set_hydrate_node(anchor);
diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index b1165a6e7aee..9412036becb3 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -137,6 +137,7 @@ export function hydrate(component, options) {
 		if (error !== HYDRATION_ERROR) {
 			// eslint-disable-next-line no-console
 			console.warn('Failed to hydrate: ', error);
+			console.error(error);
 		}
 
 		if (options.recover === false) {

From 7c0c71b72d272a2b2d5a1891262e967dbadd8c68 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 21:08:28 -0400
Subject: [PATCH 20/26] changeset

---
 .changeset/yellow-shrimps-provide.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/yellow-shrimps-provide.md

diff --git a/.changeset/yellow-shrimps-provide.md b/.changeset/yellow-shrimps-provide.md
new file mode 100644
index 000000000000..a29385660ad4
--- /dev/null
+++ b/.changeset/yellow-shrimps-provide.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: centralise branch management

From b9208dbe90fc5f2ac1887131f4ded42f5af141cf Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 17 Oct 2025 21:18:36 -0400
Subject: [PATCH 21/26] fix

---
 packages/svelte/src/internal/client/render.js | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/render.js b/packages/svelte/src/internal/client/render.js
index 9412036becb3..b1165a6e7aee 100644
--- a/packages/svelte/src/internal/client/render.js
+++ b/packages/svelte/src/internal/client/render.js
@@ -137,7 +137,6 @@ export function hydrate(component, options) {
 		if (error !== HYDRATION_ERROR) {
 			// eslint-disable-next-line no-console
 			console.warn('Failed to hydrate: ', error);
-			console.error(error);
 		}
 
 		if (options.recover === false) {

From d5ba3e24ba5a49c380b1a42f571bc03d51c2381a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 19 Oct 2025 09:32:18 -0400
Subject: [PATCH 22/26] preserve newer batches

---
 .../src/internal/client/dom/blocks/branches.js      | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 338c70a13ec8..9f7cd9dca923 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -79,19 +79,22 @@ export class BranchManager {
 			}
 		}
 
-		this.#batches.delete(batch);
-
 		for (const [b, k] of this.#batches) {
-			if (b === batch) break;
+			this.#batches.delete(b);
+
+			if (b === batch) {
+				// keep values for newer batches
+				break;
+			}
 
 			const offscreen = this.#offscreen.get(k);
 
 			if (offscreen) {
+				// for older batches, destroy offscreen effects
+				// as they will never be committed
 				destroy_effect(offscreen.effect);
 				this.#offscreen.delete(k);
 			}
-
-			this.#batches.delete(b);
 		}
 
 		// outro/destroy effects

From d331f66321a50f1dd62b8fc26ed56a5a0640a9a5 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 19 Oct 2025 09:33:19 -0400
Subject: [PATCH 23/26] add comment

---
 packages/svelte/src/internal/client/dom/blocks/branches.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 9f7cd9dca923..0b4a9f2d6188 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -97,8 +97,9 @@ export class BranchManager {
 			}
 		}
 
-		// outro/destroy effects
+		// outro/destroy all onscreen effects...
 		for (const [k, effect] of this.#onscreen) {
+			// ...except the one that was just committed
 			if (k === key) continue;
 
 			const on_destroy = () => {

From ff300d570f771e63b94c2431e6f9a28ebee2d0f0 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 20 Oct 2025 07:42:15 -0400
Subject: [PATCH 24/26] add comment

---
 packages/svelte/src/internal/client/dom/blocks/await.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/await.js b/packages/svelte/src/internal/client/dom/blocks/await.js
index 7fe04934bc60..bac01e4c3393 100644
--- a/packages/svelte/src/internal/client/dom/blocks/await.js
+++ b/packages/svelte/src/internal/client/dom/blocks/await.js
@@ -72,7 +72,8 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 				restore();
 
 				if (hydrating) {
-					// we want to restore everything _except_ this
+					// `restore()` could set `hydrating` to `true`, which we very much
+					// don't want — we want to restore everything _except_ this
 					set_hydrating(false);
 				}
 

From 6aeaa38950ced8458282d9a0299f13373fa9ff21 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 20 Oct 2025 07:42:24 -0400
Subject: [PATCH 25/26] no longer necessary apparently?

---
 packages/svelte/src/internal/client/reactivity/async.js | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 76ade0950462..1d408744fdfc 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -233,11 +233,8 @@ export async function async_body(anchor, fn) {
 		next_hydrate_node = skip_nodes(false);
 	}
 
-	var target = create_text();
-	anchor.before(target);
-
 	try {
-		var promise = fn(target);
+		var promise = fn(anchor);
 	} finally {
 		if (next_hydrate_node) {
 			set_hydrate_node(next_hydrate_node);
@@ -247,7 +244,6 @@ export async function async_body(anchor, fn) {
 
 	try {
 		await promise;
-		target.remove();
 	} catch (error) {
 		if (!aborted(active)) {
 			invoke_error_boundary(error, active);

From 4efcc529e21c92f97746c599dec301770627f78f Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 20 Oct 2025 07:48:21 -0400
Subject: [PATCH 26/26] move legacy logic to key block

---
 .../src/internal/client/dom/blocks/branches.js       |  7 -------
 .../svelte/src/internal/client/dom/blocks/key.js     | 12 +++++++++++-
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/branches.js b/packages/svelte/src/internal/client/dom/blocks/branches.js
index 0b4a9f2d6188..827f9f44faf4 100644
--- a/packages/svelte/src/internal/client/dom/blocks/branches.js
+++ b/packages/svelte/src/internal/client/dom/blocks/branches.js
@@ -32,8 +32,6 @@ export class BranchManager {
 	/** @type {Map<Key, Branch>} */
 	#offscreen = new Map();
 
-	#legacy = !is_runes();
-
 	/**
 	 * Whether to pause (i.e. outro) on change, or destroy immediately.
 	 * This is necessary for `<svelte:element>`
@@ -137,11 +135,6 @@ export class BranchManager {
 		var batch = /** @type {Batch} */ (current_batch);
 		var defer = should_defer_append();
 
-		// key blocks in Svelte <5 had stupid semantics
-		if (this.#legacy && key !== null && typeof key === 'object') {
-			key = {};
-		}
-
 		if (fn && !this.#onscreen.has(key) && !this.#offscreen.has(key)) {
 			if (defer) {
 				var fragment = document.createDocumentFragment();
diff --git a/packages/svelte/src/internal/client/dom/blocks/key.js b/packages/svelte/src/internal/client/dom/blocks/key.js
index 143b225d7f4d..849b1c24472c 100644
--- a/packages/svelte/src/internal/client/dom/blocks/key.js
+++ b/packages/svelte/src/internal/client/dom/blocks/key.js
@@ -1,4 +1,5 @@
 /** @import { TemplateNode } from '#client' */
+import { is_runes } from '../../context.js';
 import { block } from '../../reactivity/effects.js';
 import { hydrate_next, hydrating } from '../hydration.js';
 import { BranchManager } from './branches.js';
@@ -17,7 +18,16 @@ export function key(node, get_key, render_fn) {
 
 	var branches = new BranchManager(node);
 
+	var legacy = !is_runes();
+
 	block(() => {
-		branches.ensure(get_key(), render_fn);
+		var key = get_key();
+
+		// key blocks in Svelte <5 had stupid semantics
+		if (legacy && key !== null && typeof key === 'object') {
+			key = /** @type {V} */ ({});
+		}
+
+		branches.ensure(key, render_fn);
 	});
 }