fix: guard contents updated before the guard itself

.changeset/witty-seas-learn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure guards (eg. if, each, key) run before their contents

packages/svelte/src/internal/client/reactivity/batch.js

@@ -601,15 +601,32 @@ function flush_queued_effects(effects) {
 			// If update_effect() has a flushSync() in it, we may have flushed another flush_queued_effects(),
 			// which already handled this logic and did set eager_block_effects to null.
 			if (eager_block_effects?.length > 0) {
-				// TODO this feels incorrect! it gets the tests passing
 				old_values.clear();
 
+				/** @type {Array<{ effect: Effect; depth: number }>} */
+				const effects_with_depth = [];
 				for (const e of eager_block_effects) {
-					update_effect(e);
+					// Skip eager effects that have already been unmounted
+					if ((e.f & (DESTROYED | INERT)) !== 0) continue;
+
+					let depth = 0;
+					let ancestor = e.parent;
+					while (ancestor !== null) {
+						depth++;
+						ancestor = ancestor.parent;
+					}
+
+					effects_with_depth.push({ effect: e, depth });
 				}
 
-				eager_block_effects = [];
+				effects_with_depth.sort((a, b) => a.depth - b.depth);
+
+				for (const { effect } of effects_with_depth) {
+					update_effect(effect);
+				}
 			}
+
+			eager_block_effects = [];
 		}
 	}
 

packages/svelte/tests/runtime-runes/samples/guard-else-effect/_config.js

@@ -0,0 +1,22 @@
+import { expect, vi } from 'vitest';
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+const trackBranch = vi.fn();
+
+export default test({
+	mode: ['client'],
+	props: { trackBranch: trackBranch },
+	async test({ target }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		flushSync(() => button?.click());
+		flushSync(() => button?.click());
+		flushSync(() => button?.click());
+
+		expect(trackBranch).toHaveBeenCalledWith('one');
+		expect(trackBranch).toHaveBeenCalledWith('two');
+		expect(trackBranch).not.toHaveBeenCalledWith('else');
+	}
+});

packages/svelte/tests/runtime-runes/samples/guard-else-effect/main.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+	import {untrack} from 'svelte';
+
+	const {trackBranch}:{trackBranch: (branch:string)=>void} = $props();
+
+	let b = $state(false);
+	let v = $state("two");
+
+
+	$effect(() => {
+		v = b ? "one" : "two";
+	})
+</script>
+
+
+<button onclick={() => b = !b}>Trigger</button>
+
+
+{#if v === "one"}
+	<div>if1 matched! {trackBranch('one')}</div>
+{:else if v === "two"}
+	<div>if2 matched! {trackBranch('two')}</div>
+{:else}
+	<div>nothing matched {trackBranch('else')}</div>
+{/if}

packages/svelte/tests/runtime-runes/samples/guard-if-nested/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	mode: ['client'],
+	async test({ target, assert }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+
+		assert.equal(target.textContent?.trim(), 'Trigger');
+	}
+});

packages/svelte/tests/runtime-runes/samples/guard-if-nested/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let centerRow = $state({ nested: { optional: 2, required: 3 } },
+		);
+
+	let someChange = $state(false);
+	$effect(() => {
+		if (someChange) centerRow = undefined;
+	});
+</script>
+
+	{#if centerRow?.nested}
+		{#if centerRow?.nested?.optional != undefined && centerRow.nested.optional > 0}
+			op: {centerRow.nested.optional}<br />
+		{:else}
+			req: {centerRow.nested.required}<br />
+		{/if}
+	{/if}
+
+<button onclick={() => (someChange = true)}>Trigger</button>