fix: never use html optimization for mustache tags in hydration mode

.changeset/fair-geese-repeat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: never use html optimization for mustache tags in hydration mode

packages/svelte/src/compiler/compile/Component.js

@@ -1632,11 +1632,9 @@ export default class Component {
 		unsorted_reactive_declarations.forEach(add_declaration);
 	}
 	check_if_tags_content_dynamic() {
-		this.tags.forEach(
-			/** @param {any} tag */ (tag) => {
-				tag.check_if_content_dynamic();
-			}
-		);
+		this.tags.forEach((tag) => {
+			tag.check_if_content_dynamic();
+		});
 	}
 
 	/**

packages/svelte/src/compiler/compile/nodes/Element.js

@@ -1464,6 +1464,15 @@ export default class Element extends Node {
 			this.children.length > 0
 		);
 	}
+	get can_optimise_hydration() {
+		// In contrast to normal html string optimization, we also bail in case of mustache tags even
+		// if they seem to contain static content. This is because we cannot know whether that static
+		// value is different between client and server builds, e.g. {browser ? 'hi' : 'bye'} which
+		// becomes {'hi'} and {'bye'} respectively.
+		const is_static_text_content =
+			this.is_static_content && this.children.every((node) => node.type === 'Text');
+		return this.can_optimise_to_html_string && (this.can_use_innerhtml || is_static_text_content);
+	}
 	hash() {
 		return `svelte-${hash(this.component.source.slice(this.start, this.end))}`;
 	}

packages/svelte/src/compiler/compile/render_dom/wrappers/Element/index.js

@@ -458,13 +458,13 @@ export default class ElementWrapper extends Wrapper {
 		block.add_variable(node);
 		const render_statement = this.get_render_statement(block);
 		block.chunks.create.push(b`${node} = ${render_statement};`);
-		const { can_use_textcontent, can_optimise_to_html_string } = this.node;
+		const { can_use_textcontent, can_optimise_to_html_string, can_optimise_hydration } = this.node;
 		if (hydratable) {
 			if (parent_nodes) {
 				block.chunks.claim.push(b`
-					${node} = ${this.get_claim_statement(block, parent_nodes, can_optimise_to_html_string)};
+					${node} = ${this.get_claim_statement(block, parent_nodes, can_optimise_hydration)};
 				`);
-				if (!can_optimise_to_html_string && !this.void && this.node.children.length > 0) {
+				if (!can_optimise_hydration && !this.void && this.node.children.length > 0) {
 					block.chunks.claim.push(b`
 						var ${nodes} = ${children};
 					`);
@@ -500,7 +500,7 @@ export default class ElementWrapper extends Wrapper {
 		}
 		// insert static children with textContent or innerHTML
 		// skip textcontent for <template>.  append nodes to TemplateElement.content instead
-		if (can_optimise_to_html_string) {
+		if (can_optimise_to_html_string && (!hydratable || can_optimise_hydration)) {
 			if (this.fragment.nodes.length === 1 && this.fragment.nodes[0].node.type === 'Text') {
 				/** @type {import('estree').Node} */
 				let text = string_literal(
@@ -579,7 +579,7 @@ export default class ElementWrapper extends Wrapper {
 		this.add_classes(block);
 		this.add_styles(block);
 		this.add_manual_style_scoping(block);
-		if (nodes && hydratable && !this.void && !can_optimise_to_html_string) {
+		if (nodes && hydratable && !this.void && !can_optimise_hydration) {
 			block.chunks.claim.push(
 				b`${this.node.children.length > 0 ? nodes : children}.forEach(@detach);`
 			);

packages/svelte/src/compiler/compile/render_ssr/handlers/Element.js

@@ -184,7 +184,7 @@ export default function (node, renderer, options) {
 		}
 	});
 	if (options.hydratable) {
-		if (node.can_optimise_to_html_string && !options.has_added_svelte_hash) {
+		if (node.can_optimise_hydration && !options.has_added_svelte_hash) {
 			renderer.add_string(` data-svelte-h="${node.hash()}"`);
 			options = { ...options, has_added_svelte_hash: true };
 		}

packages/svelte/test/hydration/samples/repairs-apparent-static-content/_after.html

@@ -0,0 +1 @@
+<h1>Hello client!</h1>

packages/svelte/test/hydration/samples/repairs-apparent-static-content/_before.html

@@ -0,0 +1 @@
+<h1 data-svelte-h="svelte-1oz2hyi">Hello world!</h1>

packages/svelte/test/hydration/samples/repairs-apparent-static-content/_config.js

@@ -0,0 +1,7 @@
+export default {
+	snapshot(target) {
+		return {
+			h1: target.querySelector('h1')
+		};
+	}
+};

packages/svelte/test/hydration/samples/repairs-apparent-static-content/main.svelte

@@ -0,0 +1 @@
+<h1>Hello {'client'}!</h1>