From d2ec9dbebf66ea555fcf2c1f6faa26fa562f62b6 Mon Sep 17 00:00:00 2001
From: paoloricciuti <ricciutipaolo@gmail.com>
Date: Thu, 27 Aug 2026 10:43:01 +0200
Subject: [PATCH] chore: update upstream fixes to be custom renderer aware

---
 .../3-transform/client/visitors/RegularElement.js |  8 ++++++--
 .../3-transform/client/visitors/shared/element.js |  3 ++-
 .../svelte/src/internal/client/dom/blocks/slot.js |  6 +++---
 .../internal/client/dom/elements/attributes.js    | 13 +++++++------
 .../client/dom/elements/bindings/select.js        | 13 +++++++++++--
 .../samples/default-value-spread/_config.js       | 15 +++++++++++++++
 .../samples/default-value-spread/main.svelte      |  5 +++++
 7 files changed, 49 insertions(+), 14 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
index 71b23b2ea0..9fd8c08881 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
@@ -234,7 +234,11 @@ export function RegularElement(node, context) {
 
 			// `<select defaultValue>` needs the options to exist before it can mark one
 			// as selected, so it is handled after the children, alongside `value`
-			if (node.name === 'select' && get_attribute_name(node, attribute) === 'defaultValue') {
+			if (
+				!custom_renderer &&
+				node.name === 'select' &&
+				get_attribute_name(node, attribute) === 'defaultValue'
+			) {
 				continue;
 			}
 
@@ -538,7 +542,7 @@ export function RegularElement(node, context) {
 
 	// deferred from the attribute loop above, so that the options it selects from
 	// have been created and had their values assigned
-	if (!has_spread && name === 'select') {
+	if (!custom_renderer && !has_spread && name === 'select') {
 		const default_value = /** @type {AST.Attribute[]} */ (attributes).find(
 			(attribute) => get_attribute_name(node, attribute) === 'defaultValue'
 		);
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
index 332e1d791a..c66d67c523 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js
@@ -30,7 +30,8 @@ export function build_attribute_effect(
 ) {
 	/** @type {ObjectExpression['properties']} */
 	const values = [];
-	const is_select = element.type === 'RegularElement' && element.name === 'select';
+	const is_select =
+		!custom_renderer && element.type === 'RegularElement' && element.name === 'select';
 
 	const memoizer = new Memoizer();
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/slot.js b/packages/svelte/src/internal/client/dom/blocks/slot.js
index 491f04a5ea..71c7db9415 100644
--- a/packages/svelte/src/internal/client/dom/blocks/slot.js
+++ b/packages/svelte/src/internal/client/dom/blocks/slot.js
@@ -1,5 +1,5 @@
 import { hydrate_next, hydrating } from '../hydration.js';
-import { create_element, create_text } from '../operations.js';
+import { append_child, create_element, create_text, set_attribute } from '../operations.js';
 import { append } from '../template.js';
 
 /**
@@ -18,13 +18,13 @@ export function slot(anchor, $$props, name, slot_props, fallback_fn) {
 	// Use the stored reference because the shadow root may be closed.
 	if ($$props.$$host?.$$shadowRoot) {
 		const element = create_element('slot');
-		if (name !== 'default') element.name = name;
+		if (name !== 'default') set_attribute(element, 'name', name);
 
 		append(anchor, element);
 
 		if (fallback_fn !== null) {
 			const fallback_anchor = create_text();
-			element.append(fallback_anchor);
+			append_child(element, fallback_anchor);
 			fallback_fn(fallback_anchor);
 		}
 
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 6e77dee026..d881658bfb 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -111,9 +111,10 @@ export function set_value(element, value) {
 			(attributes.value =
 				// treat null and undefined the same for the initial value
 				value ?? undefined) ||
-		// @ts-expect-error
 		// `progress` elements always need their value set when it's `0`
-		(element.value === value && (value !== 0 || node_name(element) !== PROGRESS_TAG))
+		(current_renderer == null &&
+			/** @type {any} */ (element).value === value &&
+			(value !== 0 || node_name(element) !== PROGRESS_TAG))
 	) {
 		return;
 	}
@@ -368,8 +369,7 @@ function set_attributes(
 		}
 
 		if (key === 'class') {
-			var is_html =
-				element.namespaceURI === 'http://www.w3.org/1999/xhtml' && current_renderer != null;
+			var is_html = current_renderer == null && element.namespaceURI === NAMESPACE_HTML;
 			set_class(element, is_html, value, css_hash, prev?.[CLASS], next[CLASS]);
 			current[key] = value;
 			current[CLASS] = next[CLASS];
@@ -619,18 +619,19 @@ function get_attributes(element) {
 	return /** @type {Record<string | symbol, unknown>} **/ (
 		/** @type {any} */ (element)[ATTRIBUTES_CACHE] ??= {
 			[IS_CUSTOM_ELEMENT]: (node_name(element) ?? '').includes('-'),
-			[IS_HTML]: element.namespaceURI === NAMESPACE_HTML
+			[IS_HTML]: current_renderer == null && element.namespaceURI === NAMESPACE_HTML
 		}
 	);
 }
 
 /** @type {Map<string, Set<string>>} */
 var setters_cache = new Map();
+var empty_setters = new Set();
 
 /** @param {Element} element */
 function get_setters(element) {
 	// if we have a custom renderer we just skip the check all together
-	if (current_renderer) return [];
+	if (current_renderer) return empty_setters;
 	var cache_key = get_attribute(element, 'is') || (node_name(element) ?? '');
 	var setters = setters_cache.get(cache_key);
 	if (setters) return setters;
diff --git a/packages/svelte/src/internal/client/dom/elements/bindings/select.js b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
index cac7531ed2..9a9b40db19 100644
--- a/packages/svelte/src/internal/client/dom/elements/bindings/select.js
+++ b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
@@ -5,6 +5,11 @@ import { is_array } from '../../../../shared/utils.js';
 import * as w from '../../../warnings.js';
 import { Batch, current_batch, previous_batch } from '../../../reactivity/batch.js';
 import { async_mode_flag } from '../../../../flags/index.js';
+import {
+	has_attribute,
+	remove_attribute,
+	set_attribute as set_attribute_op
+} from '../../operations.js';
 
 /**
  * Sets the `selected` attribute on an option so form reset can restore it.
@@ -13,9 +18,13 @@ import { async_mode_flag } from '../../../../flags/index.js';
  */
 export function set_selected(option, selected) {
 	if (selected) {
-		if (!option.hasAttribute('selected')) option.setAttribute('selected', '');
+		// The selected option could've changed via user selection, and
+		// setting the value without this check would set it back.
+		if (!has_attribute(option, 'selected')) {
+			set_attribute_op(option, 'selected', '');
+		}
 	} else {
-		option.removeAttribute('selected');
+		remove_attribute(option, 'selected');
 	}
 }
 
diff --git a/packages/svelte/tests/custom-renderers/samples/default-value-spread/_config.js b/packages/svelte/tests/custom-renderers/samples/default-value-spread/_config.js
index e5fd87d389..2a9e3201ce 100644
--- a/packages/svelte/tests/custom-renderers/samples/default-value-spread/_config.js
+++ b/packages/svelte/tests/custom-renderers/samples/default-value-spread/_config.js
@@ -9,9 +9,18 @@ export default test({
 		const button = target.children.find(
 			(/** @type {any} */ n) => n.type === 'element' && n.name === 'button'
 		);
+		const select = target.children.find(
+			(/** @type {any} */ n) => n.type === 'element' && n.name === 'select'
+		);
+		const selected_option = select.children.find(
+			(/** @type {any} */ n) =>
+				n.type === 'element' && n.name === 'option' && n.attributes['value'] === 'other'
+		);
 
 		assert.equal(inputs.length, 4);
 		assert.ok(button);
+		assert.ok(select);
+		assert.ok(selected_option);
 
 		// Input 1: direct defaultValue attribute
 		const input_direct_val = inputs[0];
@@ -52,6 +61,9 @@ export default test({
 			'defaultChecked via spread should go through renderer.setAttribute, not element.defaultChecked'
 		);
 
+		assert.equal(select.attributes['defaultValue'], 'default_val');
+		assert.equal(selected_option.attributes['selected'], '');
+
 		// --- After update ---
 		dispatch_event(button, 'click');
 		flushSync();
@@ -75,5 +87,8 @@ export default test({
 			undefined,
 			'updated defaultChecked=false via spread should go through renderer.removeAttribute'
 		);
+
+		assert.equal(select.attributes['defaultValue'], 'new_default');
+		assert.equal(selected_option.attributes['selected'], undefined);
 	}
 });
diff --git a/packages/svelte/tests/custom-renderers/samples/default-value-spread/main.svelte b/packages/svelte/tests/custom-renderers/samples/default-value-spread/main.svelte
index c24601749e..3b5b4c40ca 100644
--- a/packages/svelte/tests/custom-renderers/samples/default-value-spread/main.svelte
+++ b/packages/svelte/tests/custom-renderers/samples/default-value-spread/main.svelte
@@ -10,6 +10,11 @@
 <input type="checkbox" checked defaultChecked={default_chk} />
 <input type="checkbox" checked {...spread_chk} />
 
+<select defaultValue={default_val}>
+	<option value="default_val">Default</option>
+	<option value="other" selected={default_chk}>Other</option>
+</select>
+
 <button onclick={() => {
 	default_val = "new_default";
 	default_chk = false;