fix: make snippet effects transparent for transitions

.changeset/lazy-knives-happen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make snippet effects transparent for transitions

packages/svelte/src/internal/client/constants.js

@@ -10,7 +10,9 @@ export const DIRTY = 1 << 9;
 export const MAYBE_DIRTY = 1 << 10;
 export const INERT = 1 << 11;
 export const DESTROYED = 1 << 12;
-export const IS_ELSEIF = 1 << 13;
-export const EFFECT_RAN = 1 << 14;
+export const EFFECT_RAN = 1 << 13;
+
+/** 'Transparent' effects do not create a transition boundary */
+export const EFFECT_TRANSPARENT = 1 << 14;
 
 export const STATE_SYMBOL = Symbol('$state');

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

@@ -1,4 +1,4 @@
-import { IS_ELSEIF } from '../../constants.js';
+import { EFFECT_TRANSPARENT } from '../../constants.js';
 import { hydrate_nodes, hydrating, set_hydrating } from '../hydration.js';
 import { remove } from '../reconciler.js';
 import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
@@ -79,6 +79,6 @@ export function if_block(
 	});
 
 	if (elseif) {
-		effect.f |= IS_ELSEIF;
+		effect.f |= EFFECT_TRANSPARENT;
 	}
 }

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

@@ -1,3 +1,4 @@
+import { EFFECT_TRANSPARENT } from '../../constants.js';
 import { branch, render_effect } from '../../reactivity/effects.js';
 
 /**
@@ -11,11 +12,13 @@ export function snippet(get_snippet, node, ...args) {
 	/** @type {SnippetFn | null | undefined} */
 	var snippet;
 
-	render_effect(() => {
+	var effect = render_effect(() => {
 		if (snippet === (snippet = get_snippet())) return;
 
 		if (snippet) {
 			branch(() => /** @type {SnippetFn} */ (snippet)(node, ...args));
 		}
 	});
+
+	effect.f |= EFFECT_TRANSPARENT;
 }

packages/svelte/src/internal/client/dom/elements/transitions.js

@@ -7,7 +7,7 @@ import { should_intro } from '../../render.js';
 import { is_function } from '../../utils.js';
 import { current_each_item } from '../blocks/each.js';
 import { TRANSITION_GLOBAL, TRANSITION_IN, TRANSITION_OUT } from '../../../../constants.js';
-import { EFFECT_RAN } from '../../constants.js';
+import { BLOCK_EFFECT, EFFECT_RAN } from '../../constants.js';
 
 /**
  * @template T
@@ -212,6 +212,11 @@ export function transition(flags, element, get_fn, get_params) {
 	if (is_intro && should_intro) {
 		var parent = /** @type {import('#client').Effect} */ (e.parent);
 
+		// e.g snippets are implemented as render effects — keep going until we find the parent block
+		while ((parent.f & BLOCK_EFFECT) === 0 && parent.parent) {
+			parent = parent.parent;
+		}
+
 		if (is_global || (parent.f & EFFECT_RAN) !== 0) {
 			effect(() => {
 				untrack(() => transition.in());

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

@@ -24,7 +24,7 @@ import {
 	EFFECT_RAN,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	IS_ELSEIF
+	EFFECT_TRANSPARENT
 } from '../constants.js';
 import { set } from './sources.js';
 import { remove } from '../dom/reconciler.js';
@@ -345,7 +345,7 @@ export function pause_children(effect, transitions, local) {
 
 	while (child !== null) {
 		var sibling = child.next;
-		var transparent = (child.f & IS_ELSEIF) !== 0 || (child.f & BRANCH_EFFECT) !== 0;
+		var transparent = (child.f & EFFECT_TRANSPARENT) !== 0 || (child.f & BRANCH_EFFECT) !== 0;
 		// TODO we don't need to call pause_children recursively with a linked list in place
 		// it's slightly more involved though as we have to account for `transparent` changing
 		// through the tree.
@@ -381,7 +381,7 @@ function resume_children(effect, local) {
 
 	while (child !== null) {
 		var sibling = child.next;
-		var transparent = (child.f & IS_ELSEIF) !== 0 || (child.f & BRANCH_EFFECT) !== 0;
+		var transparent = (child.f & EFFECT_TRANSPARENT) !== 0 || (child.f & BRANCH_EFFECT) !== 0;
 		// TODO we don't need to call resume_children recursively with a linked list in place
 		// it's slightly more involved though as we have to account for `transparent` changing
 		// through the tree.

packages/svelte/tests/runtime-runes/samples/transition-snippet/Container.svelte

@@ -0,0 +1,12 @@
+<script>
+	/** @type {{ children: import('svelte').Snippet }} */
+	let { children } = $props();
+
+	let visible = $state(false);
+</script>
+
+<button onclick={() => visible = !visible}>toggle</button>
+
+{#if visible}
+	{@render children()}
+{/if}

packages/svelte/tests/runtime-runes/samples/transition-snippet/_config.js

@@ -0,0 +1,20 @@
+import { flushSync } from 'svelte';
+import { ok, test } from '../../test';
+
+export default test({
+	test({ assert, target, raf }) {
+		const button = target.querySelector('button');
+		ok(button);
+
+		flushSync(() => button.click());
+		raf.tick(50);
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button><p style="opacity: 0.5;">hello</p>');
+
+		flushSync(() => button.click());
+		raf.tick(75);
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>toggle</button><p style="opacity: 0.25;">hello</p>'
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/transition-snippet/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	import { fade } from 'svelte/transition';
+	import { linear } from 'svelte/easing';
+	import Container from './Container.svelte';
+</script>
+
+<Container>
+	<p style="opacity: 1" transition:fade={{ duration: 100, easing: linear }}>hello</p>
+</Container>