fix: reset effects inside skipped branches (#17581)

* fix: prevent reactivity loss during fork

fixes #17197, fixes #17304, fixes #17301, fixes #17309

* add samples

* add changeset

* fix var casing in tests

* don't remove reactions during fork

* add sample for derived dep tracking in fork

* fix sample type check error

* set derived.v on first eval in fork

* add sample for derived.v remaining UNINITIALIZED

* lost current_batch import in runtime.js

* Delete how

* Update packages/svelte/src/internal/client/reactivity/deriveds.js

* delete test in favour of #17577

* extract runtime.js changes into separate PR

* alternative approach

* revert

* clear skipped branches when deferring

* fix

* fix

* changeset

* rename test

* update test

* unused test

---------

Co-authored-by: David Roizenman <david@hmnd.io>
Co-authored-by: Paolo Ricciuti <ricciutipaolo@gmail.com>
Co-authored-by: Tee Ming <chewteeming01@gmail.com>

.changeset/common-boats-travel.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reset effects inside skipped branches

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

@@ -171,6 +171,10 @@ export class Batch {
 		if (this.is_deferred()) {
 			this.#defer_effects(render_effects);
 			this.#defer_effects(effects);
+
+			for (const e of this.skipped_effects) {
+				reset_branch(e);
+			}
 		} else {
 			// append/remove branches
 			for (const fn of this.#commit_callbacks) fn();
@@ -881,6 +885,26 @@ export function eager(fn) {
 	return value;
 }
 
+/**
+ * Mark all the effects inside a skipped branch CLEAN, so that
+ * they can be correctly rescheduled later
+ * @param {Effect} effect
+ */
+function reset_branch(effect) {
+	// clean branch = nothing dirty inside, no need to traverse further
+	if ((effect.f & BRANCH_EFFECT) !== 0 && (effect.f & CLEAN) !== 0) {
+		return;
+	}
+
+	set_signal_status(effect, CLEAN);
+
+	var e = effect.first;
+	while (e !== null) {
+		reset_branch(e);
+		e = e.next;
+	}
+}
+
 /**
  * Creates a 'fork', in which state changes are evaluated but not applied to the DOM.
  * This is useful for speculatively loading data (for example) when you suspect that

packages/svelte/tests/runtime-runes/samples/async-skipped-branch/_config.js

@@ -0,0 +1,21 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork_btn, counter_btn] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			fork_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '0');
+
+		flushSync(() => {
+			counter_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-skipped-branch/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	let show = $state(false);
+	let show_async = $state(false);
+	let count = $state(0);
+</script>
+
+<button onclick={() => {
+	show = true;
+	show_async = true;
+}}>show</button>
+
+{#if show}
+	hi
+{:else}
+	{#if show || !show}
+		<button onclick={() => count++}>{count}</button>
+	{/if}
+{/if}
+
+{#if show_async}
+	{await new Promise(() => {})}
+{/if}