fix: repair each block length mismatches during hydration

.changeset/olive-socks-kick.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: repair each block length mismatches during hydration

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -1340,12 +1340,16 @@ const template_visitors = {
 			b.block(each)
 		);
 		if (node.fallback) {
+			const fallback_stmts = create_block(node, node.fallback.nodes, context);
+			fallback_stmts.unshift(
+				b.stmt(b.assignment('+=', b.id('$$payload.out'), b.literal('<!--ssr:each_else-->')))
+			);
 			state.template.push(
 				t_statement(
 					b.if(
 						b.binary('!==', b.member(array_id, b.id('length')), b.literal(0)),
 						for_loop,
-						b.block(create_block(node, node.fallback.nodes, context))
+						b.block(fallback_stmts)
 					)
 				)
 			);

packages/svelte/src/internal/client/each.js

@@ -64,6 +64,10 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 
 	/** @type {null | import('./types.js').EffectSignal} */
 	let render = null;
+
+	/** Whether or not there was a "rendered fallback but want to render items" (or vice versa) hydration mismatch */
+	let mismatch = false;
+
 	block.r =
 		/** @param {import('./types.js').Transition} transition */
 		(transition) => {
@@ -144,12 +148,30 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 				: maybe_array == null
 					? []
 					: Array.from(maybe_array);
+
 			if (key_fn !== null) {
 				keys = array.map(key_fn);
 			} else if ((flags & EACH_KEYED) === 0) {
 				array.map(no_op);
 			}
+
 			const length = array.length;
+
+			if (current_hydration_fragment !== null) {
+				const is_each_else_comment =
+					/** @type {Comment} */ (current_hydration_fragment?.[0])?.data === 'ssr:each_else';
+				// Check for hydration mismatch which can happen if the server renders the each fallback
+				// but the client has items, or vice versa. If so, remove everything inside the anchor and start fresh.
+				if ((is_each_else_comment && length) || (!is_each_else_comment && !length)) {
+					remove(/** @type {import('./types.js').TemplateNode[]} */ (current_hydration_fragment));
+					set_current_hydration_fragment(null);
+					mismatch = true;
+				} else if (is_each_else_comment) {
+					// Remove the each_else comment node or else it will confuse the subsequent hydration algorithm
+					/** @type {import('./types.js').TemplateNode[]} */ (current_hydration_fragment).shift();
+				}
+			}
+
 			if (fallback_fn !== null) {
 				if (length === 0) {
 					if (block.v.length !== 0 || render === null) {
@@ -170,6 +192,7 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 					}
 				}
 			}
+
 			if (render !== null) {
 				execute_effect(render);
 			}
@@ -180,6 +203,11 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 
 	render = render_effect(clear_each, block, true);
 
+	if (mismatch) {
+		// Set a fragment so that Svelte continues to operate in hydration mode
+		set_current_hydration_fragment([]);
+	}
+
 	push_destroy_fn(each, () => {
 		const flags = block.f;
 		const anchor_node = block.a;
@@ -287,55 +315,70 @@ function reconcile_indexed_array(
 		}
 	} else {
 		var item;
+		var is_hydrating = current_hydration_fragment !== null;
 		b_blocks = Array(b);
-		if (current_hydration_fragment !== null) {
-			/** @type {Node} */
-			var hydrating_node = current_hydration_fragment[0];
+		if (is_hydrating) {
+			// Hydrate block
+			var hydration_list = /** @type {import('./types.js').TemplateNode[]} */ (
+				current_hydration_fragment
+			);
+			var hydrating_node = hydration_list[0];
 			for (; index < length; index++) {
-				// Hydrate block
-				item = is_proxied_array ? lazy_property(array, index) : array[index];
 				var fragment = /** @type {Array<Text | Comment | Element>} */ (
 					get_hydration_fragment(hydrating_node)
 				);
 				set_current_hydration_fragment(fragment);
-				hydrating_node = /** @type {Node} */ (
+				if (!fragment) {
+					// If fragment is null, then that means that the server rendered less items than what
+					// the client code specifies -> break out and continue with client-side node creation
+					break;
+				}
+
+				item = is_proxied_array ? lazy_property(array, index) : array[index];
+				block = each_item_block(item, null, index, render_fn, flags);
+				b_blocks[index] = block;
+
+				hydrating_node = /** @type {import('./types.js').TemplateNode} */ (
 					/** @type {Node} */ (/** @type {Node} */ (fragment.at(-1)).nextSibling).nextSibling
 				);
+			}
+
+			remove_excess_hydration_nodes(hydration_list, hydrating_node);
+		}
+
+		for (; index < length; index++) {
+			if (index >= a) {
+				// Add block
+				item = is_proxied_array ? lazy_property(array, index) : array[index];
 				block = each_item_block(item, null, index, render_fn, flags);
 				b_blocks[index] = block;
+				insert_each_item_block(block, dom, is_controlled, null);
+			} else if (index >= b) {
+				// Remove block
+				block = a_blocks[index];
+				destroy_each_item_block(block, active_transitions, apply_transitions);
+			} else {
+				// Update block
+				item = array[index];
+				block = a_blocks[index];
+				b_blocks[index] = block;
+				update_each_item_block(block, item, index, flags);
 			}
-		} else {
-			for (; index < length; index++) {
-				if (index >= a) {
-					// Add block
-					item = is_proxied_array ? lazy_property(array, index) : array[index];
-					block = each_item_block(item, null, index, render_fn, flags);
-					b_blocks[index] = block;
-					insert_each_item_block(block, dom, is_controlled, null);
-				} else if (index >= b) {
-					// Remove block
-					block = a_blocks[index];
-					destroy_each_item_block(block, active_transitions, apply_transitions);
-				} else {
-					// Update block
-					item = array[index];
-					block = a_blocks[index];
-					b_blocks[index] = block;
-					update_each_item_block(block, item, index, flags);
-				}
-			}
+		}
+
+		if (is_hydrating && current_hydration_fragment === null) {
+			// Server rendered less nodes than the client -> set empty array so that Svelte continues to operate in hydration mode
+			set_current_hydration_fragment([]);
 		}
 	}
 
 	each_block.v = b_blocks;
 }
-// Reconcile arrays by the equality of the elements in the array. This algorithm
-// is based on Ivi's reconcilation logic:
-//
-// https://github.com/localvoid/ivi/blob/9f1bd0918f487da5b131941228604763c5d8ef56/packages/ivi/src/client/core.ts#L968
-//
 
 /**
+ * Reconcile arrays by the equality of the elements in the array. This algorithm
+ * is based on Ivi's reconcilation logic:
+ * https://github.com/localvoid/ivi/blob/9f1bd0918f487da5b131941228604763c5d8ef56/packages/ivi/src/client/core.ts#L968
  * @template V
  * @param {Array<V>} array
  * @param {import('./types.js').EachBlock} each_block
@@ -391,30 +434,43 @@ function reconcile_tracked_array(
 		var key;
 		var item;
 		var idx;
+		var is_hydrating = current_hydration_fragment !== null;
 		b_blocks = Array(b);
-		if (current_hydration_fragment !== null) {
+		if (is_hydrating) {
+			// Hydrate block
 			var fragment;
-
-			/** @type {Node} */
-			var hydrating_node = current_hydration_fragment[0];
+			var hydration_list = /** @type {import('./types.js').TemplateNode[]} */ (
+				current_hydration_fragment
+			);
+			var hydrating_node = hydration_list[0];
 			while (b > 0) {
-				// Hydrate block
-				idx = b_end - --b;
-				item = array[idx];
-				key = is_computed_key ? keys[idx] : item;
 				fragment = /** @type {Array<Text | Comment | Element>} */ (
 					get_hydration_fragment(hydrating_node)
 				);
 				set_current_hydration_fragment(fragment);
+				if (!fragment) {
+					// If fragment is null, then that means that the server rendered less items than what
+					// the client code specifies -> break out and continue with client-side node creation
+					break;
+				}
+
+				idx = b_end - --b;
+				item = array[idx];
+				key = is_computed_key ? keys[idx] : item;
+				block = each_item_block(item, key, idx, render_fn, flags);
+				b_blocks[idx] = block;
+
 				// Get the <!--ssr:..--> tag of the next item in the list
 				// The fragment array can be empty if each block has no content
-				hydrating_node = /** @type {Node} */ (
+				hydrating_node = /** @type {import('./types.js').TemplateNode} */ (
 					/** @type {Node} */ ((fragment.at(-1) || hydrating_node).nextSibling).nextSibling
 				);
-				block = each_item_block(item, key, idx, render_fn, flags);
-				b_blocks[idx] = block;
 			}
-		} else if (a === 0) {
+
+			remove_excess_hydration_nodes(hydration_list, hydrating_node);
+		}
+
+		if (a === 0) {
 			// Create new blocks
 			while (b > 0) {
 				idx = b_end - --b;
@@ -546,11 +602,30 @@ function reconcile_tracked_array(
 				}
 			}
 		}
+
+		if (is_hydrating && current_hydration_fragment === null) {
+			// Server rendered less nodes than the client -> set empty array so that Svelte continues to operate in hydration mode
+			set_current_hydration_fragment([]);
+		}
 	}
 
 	each_block.v = b_blocks;
 }
 
+/**
+ * The server could have rendered more list items than the client specifies.
+ * In that case, we need to remove the remaining server-rendered nodes.
+ * @param {import('./types.js').TemplateNode[]} hydration_list
+ * @param {import('./types.js').TemplateNode | null} next_node
+ */
+function remove_excess_hydration_nodes(hydration_list, next_node) {
+	if (next_node === null) return;
+	var idx = hydration_list.indexOf(next_node);
+	if (idx !== -1 && hydration_list.length > idx + 1) {
+		remove(hydration_list.slice(idx));
+	}
+}
+
 /**
  * Longest Increased Subsequence algorithm
  * @param {Int32Array} a

packages/svelte/src/internal/client/hydration.js

@@ -2,11 +2,11 @@
 
 import { schedule_task } from './runtime.js';
 
-/** @type {null | Array<Text | Comment | Element>} */
+/** @type {null | Array<import('./types.js').TemplateNode>} */
 export let current_hydration_fragment = null;
 
 /**
- * @param {null | Array<Text | Comment | Element>} fragment
+ * @param {null | Array<import('./types.js').TemplateNode>} fragment
  * @returns {void}
  */
 export function set_current_hydration_fragment(fragment) {
@@ -16,10 +16,10 @@ export function set_current_hydration_fragment(fragment) {
 /**
  * Returns all nodes between the first `<!--ssr:...-->` comment tag pair encountered.
  * @param {Node | null} node
- * @returns {Array<Text | Comment | Element> | null}
+ * @returns {Array<import('./types.js').TemplateNode> | null}
  */
 export function get_hydration_fragment(node) {
-	/** @type {Array<Text | Comment | Element>} */
+	/** @type {Array<import('./types.js').TemplateNode>} */
 	const fragment = [];
 
 	/** @type {null | Node} */

packages/svelte/tests/hydration/samples/each-block-fallback-mismatch/_after.html

@@ -0,0 +1,2 @@
+<!--ssr:0--><!--ssr:1--><p>a</p><!--ssr:1-->
+<!--ssr:2--><p>empty</p><!--ssr:2--><!--ssr:0-->

packages/svelte/tests/hydration/samples/each-block-fallback-mismatch/_before.html

@@ -0,0 +1,2 @@
+<!--ssr:0--><!--ssr:1--><!--ssr:each_else--><p>empty</p><!--ssr:1-->
+<!--ssr:2--><!--ssr:3--><p>a</p><!--ssr:3--><!--ssr:2--><!--ssr:0-->

packages/svelte/tests/hydration/samples/each-block-fallback-mismatch/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/hydration/samples/each-block-fallback-mismatch/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	let items1 = $state(typeof window !== 'undefined' ? [{name: 'a'}]: []);
+	let items2 = $state(typeof window === 'undefined' ? [{name: 'a'}]: []);
+</script>
+
+{#each items1 as item}
+	<p>{item.name}</p>
+{:else}
+	<p>empty</p>
+{/each}
+
+{#each items2 as item}
+	<p>{item.name}</p>
+{:else}
+	<p>empty</p>
+{/each}

packages/svelte/tests/hydration/samples/each-block-less-nodes-on-client/_after.html

@@ -0,0 +1,9 @@
+<!--ssr:0--><ul><!--ssr:1--><!--ssr:7--><li>a</li><!--ssr:7--><!--ssr:1--></ul>
+<ul><!--ssr:2--><!--ssr:9--><li>a</li><!--ssr:9--><!--ssr:2--></ul>
+<ul><!--ssr:3--><!--ssr:11--><li>a</li><!--ssr:11--><!--ssr:3--></ul>
+<!--ssr:4--><!--ssr:13--><li>a</li>
+<li>a</li><!--ssr:13--><!--ssr:4-->
+<!--ssr:5--><!--ssr:15--><li>a</li>
+<li>a</li><!--ssr:15--><!--ssr:5-->
+<!--ssr:6--><!--ssr:17--><li>a</li>
+<li>a</li><!--ssr:17--><!--ssr:6--><!--ssr:0--></div>

packages/svelte/tests/hydration/samples/each-block-less-nodes-on-client/_before.html

@@ -0,0 +1,12 @@
+<!--ssr:0--><ul><!--ssr:1--><!--ssr:7--><li>a</li><!--ssr:7--><!--ssr:8--><li>b</li><!--ssr:8--><!--ssr:1--></ul>
+<ul><!--ssr:2--><!--ssr:9--><li>a</li><!--ssr:9--><!--ssr:10--><li>b</li><!--ssr:10--><!--ssr:2--></ul>
+<ul><!--ssr:3--><!--ssr:11--><li>a</li><!--ssr:11--><!--ssr:12--><li>b</li><!--ssr:12--><!--ssr:3--></ul>
+<!--ssr:4--><!--ssr:13--><li>a</li>
+<li>a</li><!--ssr:13--><!--ssr:14--><li>b</li>
+<li>b</li><!--ssr:14--><!--ssr:4-->
+<!--ssr:5--><!--ssr:15--><li>a</li>
+<li>a</li><!--ssr:15--><!--ssr:16--><li>b</li>
+<li>b</li><!--ssr:16--><!--ssr:5-->
+<!--ssr:6--><!--ssr:17--><li>a</li>
+<li>a</li><!--ssr:17--><!--ssr:18--><li>b</li>
+<li>b</li><!--ssr:18--><!--ssr:6--><!--ssr:0--></div>

packages/svelte/tests/hydration/samples/each-block-less-nodes-on-client/_config.js

@@ -0,0 +1,15 @@
+import { assert_ok, test } from '../../test';
+
+export default test({
+	snapshot(target) {
+		const ul = target.querySelector('ul');
+		assert_ok(ul);
+		const lis = ul.querySelector('li');
+		assert_ok(lis);
+
+		return {
+			ul,
+			lis
+		};
+	}
+});