fix: allow NaN in key blocks (#17642)

* fix: allow NaN in key blocks

* lol whoops

* Update packages/svelte/src/internal/client/dom/blocks/key.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/clear-olives-share.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow `{#key NaN}`

packages/svelte/src/internal/client/dom/blocks/key.js

@@ -4,6 +4,8 @@ import { block } from '../../reactivity/effects.js';
 import { hydrate_next, hydrating } from '../hydration.js';
 import { BranchManager } from './branches.js';
 
+const NAN = Symbol('NaN');
+
 /**
  * @template V
  * @param {TemplateNode} node
@@ -23,6 +25,11 @@ export function key(node, get_key, render_fn) {
 	block(() => {
 		var key = get_key();
 
+		// NaN !== NaN, hence we do this workaround to not trigger remounts unnecessarily
+		if (key !== key) {
+			key = /** @type {any} */ (NAN);
+		}
+
 		// key blocks in Svelte <5 had stupid semantics
 		if (legacy && key !== null && typeof key === 'object') {
 			key = /** @type {V} */ ({});

packages/svelte/tests/runtime-runes/samples/key-nan/_config.js

@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: '<button>update</button><p>it rendered</p>',
+
+	test({ assert, target, logs }) {
+		assert.deepEqual(logs, ['rendering']);
+
+		const btn = target.querySelector('button');
+		flushSync(() => btn?.click());
+
+		// should not re-render
+		assert.deepEqual(logs, ['rendering']);
+		assert.htmlEqual(target.innerHTML, '<button>update</button><p>it rendered</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/key-nan/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let x = $state(NaN);
+</script>
+
+<button onclick={() => x = NaN}>update</button>
+
+{#key x}
+	{console.log('rendering')}
+	<p>it rendered</p>
+{/key}