fix: ensure each block consistency to internal mutations to the collection (#13614)

* fix: ensure bind_checked defers mutation to ensure reactive graph stability

* better fix

* better fix

* better fix

* better fix

* lint

* simplify

.changeset/happy-eggs-rest.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure each block consistency to internal mutations to the collection

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

@@ -225,6 +225,14 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 			// continue in hydration mode
 			set_hydrating(true);
 		}
+
+		// When we mount the each block for the first time, the collection won't be
+		// connected to this effect as the effect hasn't finished running yet and its deps
+		// won't be assigned. However, it's possible that when reconciling the each block
+		// that a mutation occurred and it's made the collection MAYBE_DIRTY, so reading the
+		// collection again can provide consistency to the reactive graph again as the deriveds
+		// will now be `CLEAN`.
+		get_collection();
 	});
 
 	if (hydrating) {

packages/svelte/tests/runtime-runes/samples/checkbox-binding-derived/_config.js

@@ -0,0 +1,16 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		button?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>Add</button><label><input type="checkbox"></label><label><input type="checkbox"></label>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/checkbox-binding-derived/checkbox.svelte

@@ -0,0 +1,7 @@
+<script>
+  let { value = $bindable() } = $props();
+</script>
+
+<label>
+  <input type="checkbox" bind:checked={value} />
+</label>

packages/svelte/tests/runtime-runes/samples/checkbox-binding-derived/main.svelte

@@ -0,0 +1,28 @@
+<script>
+	import Checkbox from './checkbox.svelte';
+
+	let foo = $state({})
+
+	const schema = $state({
+    foo: true,
+  })
+
+	function retrieveSchema() {
+		const cloned = { ...schema }
+		for (const key of Object.keys(foo)) {
+			cloned[key] = key
+		}
+		return cloned
+	}
+
+	const keys = $derived(Object.keys(retrieveSchema()));
+	let nextKey = 1;
+</script>
+
+<button onclick={() => {
+	foo[nextKey++] = true
+}}>Add</button>
+
+{#each keys as key (key)}
+	<Checkbox bind:value={foo[key]} />
+{/each}