add test, fix

packages/svelte/src/internal/client/dom/operations.js

@@ -122,6 +122,10 @@ export function child(node, is_text) {
 		return text;
 	}
 
+	if (is_text) {
+		merge_text_nodes(/** @type {Text} */ (child));
+	}
+
 	set_hydrate_node(child);
 	return child;
 }
@@ -258,3 +262,24 @@ export function set_attribute(element, key, value = '') {
 	}
 	return element.setAttribute(key, value);
 }
+
+/**
+ * Browsers split text nodes larger than 65536 bytes when parsing.
+ * For hydration to succeed, we need to stitch them back together
+ * @param {Text} text
+ */
+export function merge_text_nodes(text) {
+	if (/** @type {string} */ (text.nodeValue).length < 65536) {
+		return;
+	}
+
+	let next = text.nextSibling;
+
+	while (next !== null && next.nodeType === TEXT_NODE) {
+		next.remove();
+
+		/** @type {string} */ (text.nodeValue) += /** @type {string} */ (next.nodeValue);
+
+		next = text.nextSibling;
+	}
+}

packages/svelte/src/internal/client/dom/template.js

@@ -9,7 +9,8 @@ import {
 	create_element,
 	create_fragment,
 	create_comment,
-	set_attribute
+	set_attribute,
+	merge_text_nodes
 } from './operations.js';
 import { create_fragment_from_html } from './reconciler.js';
 import { active_effect } from '../runtime.js';
@@ -312,20 +313,7 @@ export function text(value = '') {
 		node.before((node = create_text()));
 		set_hydrate_node(node);
 	} else {
-		// Browsers may split text nodes > 65536 characters into multiple consecutive text nodes
-		// during HTML parsing. We need to merge them into a single text node.
-		var next = get_next_sibling(node);
-		var merged_text = /** @type {Text} */ (node).textContent;
-		
-		while (next !== null && next.nodeType === TEXT_NODE) {
-			merged_text += /** @type {Text} */ (next).textContent;
-			var to_remove = next;
-			next = get_next_sibling(next);
-			to_remove.remove();
-		}
-		
-		// Update the text node with the merged content
-		/** @type {Text} */ (node).textContent = merged_text;
+		merge_text_nodes(/** @type {Text} */ (node));
 	}
 
 	assign_nodes(node, node);

packages/svelte/tests/runtime-browser/samples/hydrate-large-text-node/_config.js

@@ -0,0 +1,24 @@
+import { test } from '../../assert';
+
+// Browsers split text nodes > 65536 characters into multiple consecutive text nodes
+// during HTML parsing. This test verifies that hydration correctly merges them.
+const LARGE_TEXT = 'x'.repeat(70000);
+
+export default test({
+	mode: ['hydrate'],
+	skip_mode: ['client'],
+
+	props: {
+		text: LARGE_TEXT
+	},
+
+	async test({ assert, target }) {
+		const [p] = target.querySelectorAll('p');
+
+		// The text content should be preserved after hydration
+		assert.equal(p.textContent?.trim(), LARGE_TEXT);
+		// After hydration, there should be only one text node (plus possible comment nodes)
+		const textNodes = [...p.childNodes].filter((node) => node.nodeType === 3);
+		assert.equal(textNodes.length, 1, `Expected 1 text node, got ${textNodes.length}`);
+	}
+});

packages/svelte/tests/runtime-browser/samples/hydrate-large-text-node/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	let { text } = $props();
+</script>
+
+<p>{text}</p>

packages/svelte/tests/runtime-browser/test.ts

@@ -213,7 +213,7 @@ async function run_test(
 		}
 
 		// uncomment to see what was generated
-		// fs.writeFileSync(`${test_dir}/_actual.js`, build_result.outputFiles[0].text);
+		// fs.writeFileSync(`${test_dir}/_output/bundle-${hydrate}.js`, build_result.outputFiles[0].text);
 		const test_result = await page.evaluate(
 			build_result.outputFiles[0].text + ";test.default(document.querySelector('main'))"
 		);