breaking: preserve slots inside templates with a shadowrootmode attribute (#10721)

closes #10641

.changeset/eleven-cycles-applaud.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+breaking: preserve slots inside templates with a shadowrootmode attribute

packages/svelte/src/compiler/phases/1-parse/state/element.js

@@ -51,6 +51,23 @@ function parent_is_head(stack) {
 	return false;
 }
 
+/** @param {import('#compiler').TemplateNode[]} stack */
+function parent_is_shadowroot_template(stack) {
+	// https://developer.chrome.com/docs/css-ui/declarative-shadow-dom#building_a_declarative_shadow_root
+	let i = stack.length;
+	while (i--) {
+		if (
+			stack[i].type === 'RegularElement' &&
+			/** @type {import('#compiler').RegularElement} */ (stack[i]).attributes.some(
+				(a) => a.type === 'Attribute' && a.name === 'shadowrootmode'
+			)
+		) {
+			return true;
+		}
+	}
+	return false;
+}
+
 const regex_closing_textarea_tag = /^<\/textarea(\s[^>]*)?>/i;
 const regex_closing_comment = /-->/;
 const regex_capital_letter = /[A-Z]/;
@@ -112,7 +129,8 @@ export default function tag(parser) {
 			? 'Component'
 			: name === 'title' && parent_is_head(parser.stack)
 				? 'TitleElement'
-				: name === 'slot'
+				: // TODO Svelte 6/7: once slots are removed in favor of snippets, always keep slot as a regular element
+					name === 'slot' && !parent_is_shadowroot_template(parser.stack)
 					? 'SlotElement'
 					: 'RegularElement';
 

packages/svelte/tests/parser-modern/samples/template-shadowroot/input.svelte

@@ -0,0 +1,7 @@
+<template shadowrootmode="open">
+	<p><slot></slot></p>
+</template>
+<template>
+	<p><slot></slot></p>
+</template>
+<slot></slot>

packages/svelte/tests/parser-modern/samples/template-shadowroot/output.json

@@ -0,0 +1,160 @@
+{
+	"css": null,
+	"js": [],
+	"start": 0,
+	"end": 125,
+	"type": "Root",
+	"fragment": {
+		"type": "Fragment",
+		"nodes": [
+			{
+				"type": "RegularElement",
+				"start": 0,
+				"end": 66,
+				"name": "template",
+				"attributes": [
+					{
+						"type": "Attribute",
+						"start": 10,
+						"end": 31,
+						"name": "shadowrootmode",
+						"value": [
+							{
+								"start": 26,
+								"end": 30,
+								"type": "Text",
+								"raw": "open",
+								"data": "open"
+							}
+						]
+					}
+				],
+				"fragment": {
+					"type": "Fragment",
+					"nodes": [
+						{
+							"type": "Text",
+							"start": 32,
+							"end": 34,
+							"raw": "\n\t",
+							"data": "\n\t"
+						},
+						{
+							"type": "RegularElement",
+							"start": 34,
+							"end": 54,
+							"name": "p",
+							"attributes": [],
+							"fragment": {
+								"type": "Fragment",
+								"nodes": [
+									{
+										"type": "RegularElement",
+										"start": 37,
+										"end": 50,
+										"name": "slot",
+										"attributes": [],
+										"fragment": {
+											"type": "Fragment",
+											"nodes": [],
+											"transparent": true
+										}
+									}
+								],
+								"transparent": true
+							}
+						},
+						{
+							"type": "Text",
+							"start": 54,
+							"end": 55,
+							"raw": "\n",
+							"data": "\n"
+						}
+					],
+					"transparent": true
+				}
+			},
+			{
+				"type": "Text",
+				"start": 66,
+				"end": 67,
+				"raw": "\n",
+				"data": "\n"
+			},
+			{
+				"type": "RegularElement",
+				"start": 67,
+				"end": 111,
+				"name": "template",
+				"attributes": [],
+				"fragment": {
+					"type": "Fragment",
+					"nodes": [
+						{
+							"type": "Text",
+							"start": 77,
+							"end": 79,
+							"raw": "\n\t",
+							"data": "\n\t"
+						},
+						{
+							"type": "RegularElement",
+							"start": 79,
+							"end": 99,
+							"name": "p",
+							"attributes": [],
+							"fragment": {
+								"type": "Fragment",
+								"nodes": [
+									{
+										"type": "SlotElement",
+										"start": 82,
+										"end": 95,
+										"name": "slot",
+										"attributes": [],
+										"fragment": {
+											"type": "Fragment",
+											"nodes": [],
+											"transparent": true
+										}
+									}
+								],
+								"transparent": true
+							}
+						},
+						{
+							"type": "Text",
+							"start": 99,
+							"end": 100,
+							"raw": "\n",
+							"data": "\n"
+						}
+					],
+					"transparent": true
+				}
+			},
+			{
+				"type": "Text",
+				"start": 111,
+				"end": 112,
+				"raw": "\n",
+				"data": "\n"
+			},
+			{
+				"type": "SlotElement",
+				"start": 112,
+				"end": 125,
+				"name": "slot",
+				"attributes": [],
+				"fragment": {
+					"type": "Fragment",
+					"nodes": [],
+					"transparent": true
+				}
+			}
+		],
+		"transparent": false
+	},
+	"options": null
+}