support dynamic svelte:element namespace through xmlns attribute

.changeset/hip-pumpkins-boil.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: fall back to component namespace when not statically determinable, add way to tell `<svelte:element>` the namespace at runtime

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

@@ -2008,6 +2008,9 @@ export const template_visitors = {
 		/** @type {Array<import('#compiler').Attribute | import('#compiler').SpreadAttribute>} */
 		const attributes = [];
 
+		/** @type {import('#compiler').Attribute['value'] | undefined} */
+		let dynamic_namespace = undefined;
+
 		/** @type {import('#compiler').ClassDirective[]} */
 		const class_directives = [];
 
@@ -2036,7 +2039,11 @@ export const template_visitors = {
 
 		for (const attribute of node.attributes) {
 			if (attribute.type === 'Attribute') {
-				attributes.push(attribute);
+				if (attribute.name === 'xmlns' && !is_text_attribute(attribute)) {
+					dynamic_namespace = attribute.value;
+				} else {
+					attributes.push(attribute);
+				}
 			} else if (attribute.type === 'SpreadAttribute') {
 				attributes.push(attribute);
 			} else if (attribute.type === 'ClassDirective') {
@@ -2090,19 +2097,16 @@ export const template_visitors = {
 				}
 			})
 		);
-		context.state.init.push(
-			b.stmt(
-				b.call(
-					'$.element',
-					context.state.node,
-					get_tag,
-					node.metadata.svg ? b.true : b.false,
-					inner.length === 0
-						? /** @type {any} */ (undefined)
-						: b.arrow([element_id, b.id('$$anchor')], b.block(inner))
-				)
-			)
-		);
+
+		const args = [context.state.node, get_tag, node.metadata.svg ? b.true : b.false];
+		if (inner.length > 0) {
+			args.push(b.arrow([element_id, b.id('$$anchor')], b.block(inner)));
+		}
+		if (dynamic_namespace) {
+			if (inner.length === 0) args.push(b.id('undefined'));
+			args.push(b.thunk(serialize_attribute_value(dynamic_namespace, context)[1]));
+		}
+		context.state.init.push(b.stmt(b.call('$.element', ...args)));
 	},
 	EachBlock(node, context) {
 		const each_node_meta = node.metadata;

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -40,10 +40,11 @@ function swap_block_dom(effect, from, to) {
  * @param {Comment} anchor
  * @param {() => string} get_tag
  * @param {boolean} is_svg
- * @param {undefined | ((element: Element, anchor: Node) => void)} render_fn
+ * @param {undefined | ((element: Element, anchor: Node) => void)} render_fn,
+ * @param {undefined | (() => string)} get_namespace
  * @returns {void}
  */
-export function element(anchor, get_tag, is_svg, render_fn) {
+export function element(anchor, get_tag, is_svg, render_fn, get_namespace) {
 	const parent_effect = /** @type {import('#client').Effect} */ (current_effect);
 
 	render_effect(() => {
@@ -68,17 +69,14 @@ export function element(anchor, get_tag, is_svg, render_fn) {
 
 		block(() => {
 			const next_tag = get_tag() || null;
+			const ns = get_namespace?.() || (is_svg || next_tag === 'svg' ? namespace_svg : null);
+			// Assumption: Noone changes the namespace but not the tag (what would that even mean?)
 			if (next_tag === tag) return;
 
 			// See explanation of `each_item_block` above
 			var previous_each_item = current_each_item;
 			set_current_each_item(each_item_block);
 
-			// The namespace may not be statically known but we can't really infer it either,
-			// because on the first render on the client (without hydration) the parent will be undefined,
-			// and the element itself could be a tag that changes the namespace.
-			const ns = is_svg || next_tag === 'svg' ? namespace_svg : null;
-
 			if (effect) {
 				if (next_tag === null) {
 					// start outro

packages/svelte/tests/runtime-runes/samples/dynamic-element-dynamic-namespace/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		assert.equal(target.querySelector('path')?.namespaceURI, 'http://www.w3.org/2000/svg');
+
+		await target.querySelector('button')?.click();
+		assert.equal(target.querySelector('div')?.namespaceURI, 'http://www.w3.org/1999/xhtml');
+	}
+});

packages/svelte/tests/runtime-runes/samples/dynamic-element-dynamic-namespace/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	let tag = $state('path');
+	let xmlns = $state('http://www.w3.org/2000/svg');
+</script>
+
+<button onclick={() => {
+	tag = 'div';
+	xmlns = 'http://www.w3.org/1999/xhtml';
+}}>change</button>
+
+<!-- wrapper necessary or else jsdom says this is always an xhtml namespace -->
+<svg>
+	<svelte:element this={tag} xmlns={xmlns} />
+</svg>