fix: better sibling selector handling

.changeset/giant-plants-grin.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: keep sibling selectors when dealing with slots/render tags/`svelte:element` tags

packages/svelte/src/compiler/phases/2-analyze/css/css-prune.js

@@ -175,7 +175,13 @@ function apply_selector(relative_selectors, rule, element, stylesheet) {
 				let sibling_matched = false;
 
 				for (const possible_sibling of siblings.keys()) {
-					if (apply_selector(parent_selectors, rule, possible_sibling, stylesheet)) {
+					if (possible_sibling.type === 'RenderTag' || possible_sibling.type === 'SlotElement') {
+						// `{@render foo()}<p>foo</p>` with `:global(.x) + p` is a match
+						if (parent_selectors.length === 1 && parent_selectors[0].metadata.is_global) {
+							mark(relative_selector, element);
+							sibling_matched = true;
+						}
+					} else if (apply_selector(parent_selectors, rule, possible_sibling, stylesheet)) {
 						mark(relative_selector, element);
 						sibling_matched = true;
 					}
@@ -564,38 +570,39 @@ function get_element_parent(node) {
 function find_previous_sibling(node) {
 	/** @type {import('#compiler').SvelteNode} */
 	let current_node = node;
-	do {
-		if (current_node.type === 'SlotElement') {
-			const slot_children = current_node.fragment.nodes;
-			if (slot_children.length > 0) {
-				current_node = slot_children.slice(-1)[0]; // go to its last child first
-				continue;
-			}
-		}
-		while (
-			// @ts-expect-error TODO
-			!current_node.prev &&
-			// @ts-expect-error TODO
-			current_node.parent &&
-			// @ts-expect-error TODO
-			current_node.parent.type === 'SlotElement'
-		) {
-			// @ts-expect-error TODO
-			current_node = current_node.parent;
+
+	while (
+		// @ts-expect-error TODO
+		!current_node.prev &&
+		// @ts-expect-error TODO
+		current_node.parent?.type === 'SlotElement'
+	) {
+		// @ts-expect-error TODO
+		current_node = current_node.parent;
+	}
+
+	// @ts-expect-error
+	current_node = current_node.prev;
+
+	while (current_node?.type === 'SlotElement') {
+		const slot_children = current_node.fragment.nodes;
+		if (slot_children.length > 0) {
+			current_node = slot_children.slice(-1)[0];
+		} else {
+			break;
 		}
-		// @ts-expect-error
-		current_node = current_node.prev;
-	} while (current_node && current_node.type === 'SlotElement');
+	}
+
 	return current_node;
 }
 
 /**
  * @param {import('#compiler').SvelteNode} node
  * @param {boolean} adjacent_only
- * @returns {Map<import('#compiler').RegularElement, NodeExistsValue>}
+ * @returns {Map<import('#compiler').RegularElement | import('#compiler').SvelteElement | import('#compiler').SlotElement | import('#compiler').RenderTag, NodeExistsValue>}
  */
 function get_possible_element_siblings(node, adjacent_only) {
-	/** @type {Map<import('#compiler').RegularElement, NodeExistsValue>} */
+	/** @type {Map<import('#compiler').RegularElement | import('#compiler').SvelteElement | import('#compiler').SlotElement | import('#compiler').RenderTag, NodeExistsValue>} */
 	const result = new Map();
 
 	/** @type {import('#compiler').SvelteNode} */
@@ -618,6 +625,14 @@ function get_possible_element_siblings(node, adjacent_only) {
 			if (adjacent_only && has_definite_elements(possible_last_child)) {
 				return result;
 			}
+		} else if (
+			prev.type === 'SlotElement' ||
+			prev.type === 'RenderTag' ||
+			prev.type === 'SvelteElement'
+		) {
+			result.set(prev, NODE_PROBABLY_EXISTS);
+			// Special case: slots, render tags and svelte:element tags could resolve to no siblings,
+			// so we want to continue until we find a definite sibling even with the adjacent-only combinator
 		}
 	}
 
@@ -720,7 +735,7 @@ function get_possible_last_child(relative_selector, adjacent_only) {
 }
 
 /**
- * @param {Map<import('#compiler').RegularElement, NodeExistsValue>} result
+ * @param {Map<unknown, NodeExistsValue>} result
  * @returns {boolean}
  */
 function has_definite_elements(result) {
@@ -734,8 +749,9 @@ function has_definite_elements(result) {
 }
 
 /**
- * @param {Map<import('#compiler').RegularElement, NodeExistsValue>} from
- * @param {Map<import('#compiler').RegularElement, NodeExistsValue>} to
+ * @template T
+ * @param {Map<T, NodeExistsValue>} from
+ * @param {Map<T, NodeExistsValue>} to
  * @returns {void}
  */
 function add_to_map(from, to) {

packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		// TODO
+		// {
+		// 	code: 'css-unused-selector',
+		// 	message: 'Unused CSS selector ".a ~ .b"',
+		// 	start: { character: 111, column: 1, line: 10 },
+		// 	end: { character: 118, column: 8, line: 10 }
+		// },
+	]
+});

packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/expected.css

@@ -0,0 +1,13 @@
+
+	.before.svelte-xyz + .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .bar:where(.svelte-xyz) { color: green; }
+
+	.x + .foo.svelte-xyz { color: green; }
+	.x + .foo.svelte-xyz span:where(.svelte-xyz) { color: green; }
+	.x ~ .foo.svelte-xyz { color: green; }
+	.x ~ .foo.svelte-xyz span:where(.svelte-xyz) { color: green; }
+	.x ~ .bar.svelte-xyz { color: green; }
+
+	/* no match */
+	/* (unused) :global(.x) + .bar { color: green; }*/

packages/svelte/tests/css/samples/general-siblings-combinator-rendertag-global/input.svelte

@@ -0,0 +1,23 @@
+<div>
+	<p class="before">before</p>
+	{@render children()}
+	<p class="foo">
+		<span>foo</span>
+	</p>
+	<p class="bar">bar</p>
+</div>
+
+<style>
+	.before + .foo { color: green; }
+	.before ~ .foo { color: green; }
+	.before ~ .bar { color: green; }
+
+	:global(.x) + .foo { color: green; }
+	:global(.x) + .foo span { color: green; }
+	:global(.x) ~ .foo { color: green; }
+	:global(.x) ~ .foo span { color: green; }
+	:global(.x) ~ .bar { color: green; }
+
+	/* no match */
+	:global(.x) + .bar { color: green; }
+</style>

packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		// TODO
+		// {
+		// 	code: 'css-unused-selector',
+		// 	message: 'Unused CSS selector ".a ~ .b"',
+		// 	start: { character: 111, column: 1, line: 10 },
+		// 	end: { character: 118, column: 8, line: 10 }
+		// },
+	]
+});

packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/expected.css

@@ -0,0 +1,13 @@
+
+	.before.svelte-xyz + .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .bar:where(.svelte-xyz) { color: green; }
+
+	.x + .foo.svelte-xyz { color: green; }
+	.x + .foo.svelte-xyz span:where(.svelte-xyz) { color: green; }
+	.x ~ .foo.svelte-xyz { color: green; }
+	.x ~ .foo.svelte-xyz span:where(.svelte-xyz) { color: green; }
+	.x ~ .bar.svelte-xyz { color: green; }
+
+	/* no match */
+	/* (unused) :global(.x) + .bar { color: green; }*/

packages/svelte/tests/css/samples/general-siblings-combinator-slot-global/input.svelte

@@ -0,0 +1,23 @@
+<div>
+	<p class="before">before</p>
+	<slot></slot>
+	<p class="foo">
+		<span>foo</span>
+	</p>
+	<p class="bar">bar</p>
+</div>
+
+<style>
+	.before + .foo { color: green; }
+	.before ~ .foo { color: green; }
+	.before ~ .bar { color: green; }
+
+	:global(.x) + .foo { color: green; }
+	:global(.x) + .foo span { color: green; }
+	:global(.x) ~ .foo { color: green; }
+	:global(.x) ~ .foo span { color: green; }
+	:global(.x) ~ .bar { color: green; }
+
+	/* no match */
+	:global(.x) + .bar { color: green; }
+</style>

packages/svelte/tests/css/samples/general-siblings-combinator-svelteelement/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		// TODO
+		// {
+		// 	code: 'css-unused-selector',
+		// 	message: 'Unused CSS selector ".a ~ .b"',
+		// 	start: { character: 111, column: 1, line: 10 },
+		// 	end: { character: 118, column: 8, line: 10 }
+		// },
+	]
+});

packages/svelte/tests/css/samples/general-siblings-combinator-svelteelement/expected.css

@@ -0,0 +1,13 @@
+
+	.before.svelte-xyz + .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .foo:where(.svelte-xyz) { color: green; }
+	.before.svelte-xyz ~ .bar:where(.svelte-xyz) { color: green; }
+
+	.x.svelte-xyz + .foo:where(.svelte-xyz) { color: green; }
+	.x.svelte-xyz + .foo:where(.svelte-xyz) span:where(.svelte-xyz) { color: green; }
+	.x.svelte-xyz ~ .foo:where(.svelte-xyz) { color: green; }
+	.x.svelte-xyz ~ .foo:where(.svelte-xyz) span:where(.svelte-xyz) { color: green; }
+	.x.svelte-xyz ~ .bar:where(.svelte-xyz) { color: green; }
+
+	/* no match */
+	/* (unused) .x + .bar { color: green; }*/

packages/svelte/tests/css/samples/general-siblings-combinator-svelteelement/input.svelte

@@ -0,0 +1,27 @@
+<script>
+	let tag = 'div'
+</script>
+
+<div>
+	<p class="before">before</p>
+	<svelte:element class="x" this={tag}></svelte:element>
+	<p class="foo">
+		<span>foo</span>
+	</p>
+	<p class="bar">bar</p>
+</div>
+
+<style>
+	.before + .foo { color: green; }
+	.before ~ .foo { color: green; }
+	.before ~ .bar { color: green; }
+
+	.x + .foo { color: green; }
+	.x + .foo span { color: green; }
+	.x ~ .foo { color: green; }
+	.x ~ .foo span { color: green; }
+	.x ~ .bar { color: green; }
+
+	/* no match */
+	.x + .bar { color: green; }
+</style>