move legacy logic to key block

Author: Rich-Harris

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

@@ -32,8 +32,6 @@ export class BranchManager {
 	/** @type {Map<Key, Branch>} */
 	#offscreen = new Map();
 
-	#legacy = !is_runes();
-
 	/**
 	 * Whether to pause (i.e. outro) on change, or destroy immediately.
 	 * This is necessary for `<svelte:element>`
@@ -137,11 +135,6 @@ export class BranchManager {
 		var batch = /** @type {Batch} */ (current_batch);
 		var defer = should_defer_append();
 
-		// key blocks in Svelte <5 had stupid semantics
-		if (this.#legacy && key !== null && typeof key === 'object') {
-			key = {};
-		}
-
 		if (fn && !this.#onscreen.has(key) && !this.#offscreen.has(key)) {
 			if (defer) {
 				var fragment = document.createDocumentFragment();

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

@@ -1,4 +1,5 @@
 /** @import { TemplateNode } from '#client' */
+import { is_runes } from '../../context.js';
 import { block } from '../../reactivity/effects.js';
 import { hydrate_next, hydrating } from '../hydration.js';
 import { BranchManager } from './branches.js';
@@ -17,7 +18,16 @@ export function key(node, get_key, render_fn) {
 
 	var branches = new BranchManager(node);
 
+	var legacy = !is_runes();
+
 	block(() => {
-		branches.ensure(get_key(), render_fn);
+		var key = get_key();
+
+		// key blocks in Svelte <5 had stupid semantics
+		if (legacy && key !== null && typeof key === 'object') {
+			key = /** @type {V} */ ({});
+		}
+
+		branches.ensure(key, render_fn);
 	});
 }