fix: add already connected validation

documentation/docs/98-reference/.generated/client-errors.md

@@ -133,3 +133,9 @@ Reading state that was created inside the same derived is forbidden. Consider us
 ```
 Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```
+
+### svelte_element_already_connected
+
+```
+You can't use an HTML element as the `this` attribute of `svelte:element` if it's already connected to a document
+```

packages/svelte/messages/client-errors/errors.md

@@ -87,3 +87,7 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 ## state_unsafe_mutation
 
 > Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
+
+## svelte_element_already_connected
+
+> You can't use an HTML element as the `this` attribute of `svelte:element` if it's already connected to a document

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

@@ -23,6 +23,7 @@ import { DEV } from 'esm-env';
 import { EFFECT_TRANSPARENT } from '../../constants.js';
 import { assign_nodes } from '../template.js';
 import { is_raw_text_element } from '../../../../utils.js';
+import * as e from '../../errors.js';
 
 /**
  * @param {Comment | Element} node
@@ -70,11 +71,19 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 
 	block(() => {
 		const next_tag = get_tag() || null;
-		var ns = get_namespace ? get_namespace() : is_svg || next_tag === 'svg' ? NAMESPACE_SVG : null;
+		var ns = get_namespace
+			? get_namespace()
+			: is_svg || (typeof next_tag === 'string' ? next_tag === 'svg' : next_tag?.tagName === 'svg')
+				? NAMESPACE_SVG
+				: null;
 
 		// Assumption: Noone changes the namespace but not the tag (what would that even mean?)
 		if (next_tag === tag) return;
 
+		if (typeof next_tag !== 'string' && next_tag?.isConnected) {
+			e.svelte_element_already_connected();
+		}
+
 		// See explanation of `each_item_block` above
 		var previous_each_item = current_each_item;
 		set_current_each_item(each_item_block);

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

@@ -336,3 +336,18 @@ export function state_unsafe_mutation() {
 		throw new Error(`https://svelte.dev/e/state_unsafe_mutation`);
 	}
 }
+
+/**
+ * You can't use an HTML element as the `this` attribute of `svelte:element` if it's already connected to a document
+ * @returns {never}
+ */
+export function svelte_element_already_connected() {
+	if (DEV) {
+		const error = new Error(`svelte_element_already_connected\nYou can't use an HTML element as the \`this\` attribute of \`svelte:element\` if it's already connected to a document\nhttps://svelte.dev/e/svelte_element_already_connected`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/svelte_element_already_connected`);
+	}
+}

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

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+		assert.throws(() => {
+			flushSync(() => {
+				btn?.click();
+			});
+		}, 'svelte_element_already_connected');
+	}
+});

packages/svelte/tests/runtime-runes/samples/svelte-element-element-connected/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let div = $state(null);
+	let show = $state(false);
+</script>
+
+<div bind:this={div}></div>
+<button onclick={() => show = !show}></button>
+<svelte:element this={show && div}>
+</svelte:element>