fix: better handling of empty text node hydration

.changeset/heavy-comics-move.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: better handling of empty text node hydration

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -1127,7 +1127,7 @@ function create_block(parent, name, nodes, context) {
 			trimmed.every((node) => node.type === 'Text' || node.type === 'ExpressionTag');
 
 		if (use_space_template) {
-			// special case — we can use `$.space` instead of creating a unique template
+			// special case — we can use `$.space_frag` instead of creating a unique template
 			const id = b.id(context.state.scope.generate('text'));
 
 			process_children(trimmed, () => id, false, {
@@ -1135,7 +1135,7 @@ function create_block(parent, name, nodes, context) {
 				state
 			});
 
-			body.push(b.var(id, b.call('$.space', b.id('$$anchor'))), ...state.init);
+			body.push(b.var(id, b.call('$.space_frag', b.id('$$anchor'))), ...state.init);
 			close = b.stmt(b.call('$.close', b.id('$$anchor'), id));
 		} else {
 			/** @type {(is_text: boolean) => import('estree').Expression} */
@@ -1495,7 +1495,7 @@ function process_children(nodes, expression, is_element, { visit, state }) {
 
 			state.template.push(' ');
 
-			const text_id = get_node_id(expression(true), state, 'text');
+			const text_id = get_node_id(b.call('$.space', expression(true)), state, 'text');
 
 			const singular = b.stmt(
 				b.call(

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

@@ -206,7 +206,7 @@ const comment_template = template('<!>', true);
  * @param {Text | Comment | Element | null} anchor
  */
 /*#__NO_SIDE_EFFECTS__*/
-export function space(anchor) {
+export function space_frag(anchor) {
 	/** @type {Node | null} */
 	var node = /** @type {any} */ (open(anchor, true, space_template));
 	// if an {expression} is empty during SSR, there might be no
@@ -216,10 +216,26 @@ export function space(anchor) {
 		node = empty();
 		// @ts-ignore in this case the anchor should always be a comment,
 		// if not something more fundamental is wrong and throwing here is better to bail out early
-		anchor.parentElement.insertBefore(node, anchor);
+		anchor.before(node);
+	}
+	return node;
 }
+
+/**
+ * @param {Text | Comment | Element} anchor
+ */
+/*#__NO_SIDE_EFFECTS__*/
+export function space(anchor) {
+	// if an {expression} is empty during SSR, there might be no
+	// text node to hydrate (or an anchor comment is falsely detected instead)
+	//  — we must therefore create one
+	if (hydrating && anchor.nodeType !== 3) {
+		const node = empty()
+		anchor.before(node);
 		return node;
 	}
+	return anchor;
+}
 
 /**
  * @param {null | Text | Comment | Element} anchor

packages/svelte/tests/runtime-runes/samples/state-space/_config.js

@@ -0,0 +1,12 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<button></button>`,
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		await btn?.click();
+		assert.htmlEqual(target.innerHTML, `<button>foo</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-space/main.svelte

@@ -0,0 +1,5 @@
+<script>
+    let foo = $state();
+</script>
+
+<button on:click={() => foo = 'foo'}>{foo}</button>

packages/svelte/tests/runtime-runes/samples/state-undefined/_config.js

@@ -1,12 +1,15 @@
 import { test } from '../../test';
 
 export default test({
-	html: `<button></button>`,
+	html: `<button type="button">Update Text</button><div></div>`,
 
 	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
-		assert.htmlEqual(target.innerHTML, `<button>foo</button>`);
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button type="button">Update Text</button><div>updated</div>`
+		);
 	}
 });

packages/svelte/tests/runtime-runes/samples/state-undefined/main.svelte

@@ -1,5 +1,11 @@
 <script>
-    let foo = $state();
+	let text = $state('');
+
+	function update_text() {
+			text = 'updated';
+	}
 </script>
 
-<button on:click={() => foo = 'foo'}>{foo}</button>
+<button type="button" on:click={update_text}>Update Text</button>
+
+<div>{text}</div>

packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js

@@ -17,7 +17,7 @@ export default function Each_string_template($$anchor, $$props) {
 		1,
 		($$anchor, thing, $$index) => {
 			/* Init */
-			var text = $.space($$anchor);
+			var text = $.space_frag($$anchor);
 
 			/* Update */
 			$.text_effect(text, () => `${$.stringify($.unwrap(thing))}, `);

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/client/index.svelte.js

@@ -23,7 +23,7 @@ export default function Function_prop_no_getter($$anchor, $$props) {
 		onmouseenter: () => $.set(count, $.proxy(plusOne($.get(count)))),
 		children: ($$anchor, $$slotProps) => {
 			/* Init */
-			var text = $.space($$anchor);
+			var text = $.space_frag($$anchor);
 
 			/* Update */
 			$.text_effect(text, () => `clicks: ${$.stringify($.get(count))}`);