Make sure all data parsing or schema related checks use proper/actual root

.changelog/20260429140000_ck_20026_title_custom_roots.md

@@ -0,0 +1,9 @@
+---
+type: Fix
+scope:
+  - ckeditor5-heading
+closes:
+  - https://github.com/ckeditor/ckeditor5/issues/20026
+---
+
+The Title feature now correctly handles editor configurations where some or all roots use a custom `modelElement`. Roots that do not accept the `title` element are silently skipped at runtime, and the feature logs a single warning when no root supports the title structure.

.changelog/20260429140100_ck_20026_ghs_hgroup.md

@@ -0,0 +1,9 @@
+---
+type: Fix
+scope:
+  - ckeditor5-html-support
+closes:
+  - https://github.com/ckeditor/ckeditor5/issues/20026
+---
+
+The General HTML Support schema for the `hgroup` element no longer hardcodes `$root` as its allowed parent, so the element is correctly registered in editor configurations using a custom root `modelElement`.

.changelog/20260429140200_ck_20026_html_comments.md

@@ -0,0 +1,9 @@
+---
+type: Fix
+scope:
+  - ckeditor5-html-support
+closes:
+  - https://github.com/ckeditor/ckeditor5/issues/20026
+---
+
+The HTML comments feature no longer assumes the root model element is `$root`. Comments are now preserved in editor configurations using a custom root `modelElement`.

eslint.config.mjs

@@ -95,6 +95,11 @@ export default defineConfig( [
 			'ckeditor5-rules/validate-module-tag': 'error',
 			'ckeditor5-rules/no-default-export': 'error',
 			'ckeditor5-rules/allow-svg-imports-only-in-icons-package': 'error',
+			'ckeditor5-rules/no-literal-dollar-root': [ 'error', {
+				allowedPackages: [ 'ckeditor5-engine', 'ckeditor5-core' ],
+				allowedCalls: [ 'is' ]
+			} ],
+			'ckeditor5-rules/require-explicit-data-context': 'error',
 			'ckeditor5-rules/ckeditor-plugin-flags': [ 'error', {
 				requiredFlags: [ {
 					name: 'isOfficialPlugin',

package.json

@@ -62,9 +62,9 @@
     "date-fns": "^4.0.0",
     "esbuild": "^0.25.0",
     "eslint": "^9.34.0",
-    "eslint-config-ckeditor5": "^15.0.0",
+    "eslint-config-ckeditor5": "^15.1.0",
     "eslint-formatter-stylish": "^8.40.0",
-    "eslint-plugin-ckeditor5-rules": "^15.0.0",
+    "eslint-plugin-ckeditor5-rules": "^15.1.0",
     "estree-walker": "^3.0.3",
     "fs-extra": "^11.0.0",
     "glob": "^13.0.0",

packages/ckeditor5-core/src/editor/editor.ts

@@ -953,12 +953,18 @@ export abstract class Editor extends /* #__PURE__ */ ObservableMixin() {
 	 * Registers a given string as a root attribute key. Registered root attributes are added to
 	 * the {@link module:engine/model/schema~ModelSchema schema}.
 	 *
-	 * Note: Attributes passed in the configuration for multi-root editors
+	 * **Note:** Attributes passed in the configuration for multi-root editors
 	 * ({@link module:core/editor/editorconfig~EditorConfig#roots `config.roots.<rootName>.modelAttributes`}) or
 	 * single-root editors ({@link module:core/editor/editorconfig~EditorConfig#root `config.root.modelAttributes`})
 	 * are automatically registered when the editor is initialized. However, registering the same attribute twice
 	 * does not have any negative impact, so it is recommended to use this method in any feature that uses
 	 * root attributes.
+	 *
+	 * **Note:** Registered attributes are attached only to the generic `$root` schema element. A custom root
+	 * {@link module:core/editor/editorconfig~RootConfig#modelElement `modelElement`} must opt into the `$root`
+	 * attribute chain via `allowAttributesOf: '$root'` to inherit these attributes.
+	 * See the {@glink framework/deep-dive/schema#custom-root-elements Custom root elements} section of the
+	 * {@glink framework/deep-dive/schema Schema deep-dive} guide for more details.
 	 */
 	public registerRootAttribute( key: string ): void {
 		if ( this._registeredRootsAttributesKeys.has( key ) ) {

packages/ckeditor5-editor-multi-root/src/multirooteditor.ts

@@ -115,6 +115,7 @@ export class MultiRootEditor extends Editor {
 
 		normalizeRootsConfig( sourceElementsOrData, this.config, false );
 		normalizeRootsAttributesConfig( this.config );
+		normalizeRootEditableOptionsConfig( this.config );
 
 		if ( this.config.get( 'lazyRoots' ) ) {
 			/**
@@ -175,30 +176,12 @@ export class MultiRootEditor extends Editor {
 			}
 		}
 
-		registerAndInitializeRootConfigAttributes( this );
+		// Register `$rootEditableOptions` unconditionally, so it is always returned by `getRootAttributes()` (e.g. for RH).
+		// The value is set via `config.roots.<rootName>.modelAttributes.$rootEditableOptions` (see `normalizeRootEditableOptionsConfig`),
+		// which also makes it round-trip through RTC's initial-data path.
+		this.registerRootAttribute( '$rootEditableOptions' );
 
-		// Registering `$rootEditableOptions` attribute to make it available in the editor model.
-		// This allows to store editable options for each root in the model, and make them available on other RTC clients.
-		// We do not use `registerRootAttribute()` method here, as this attribute is used internally
-		// and should not be returned by `getRootsAttributes()` method.
-		this.editing.model.schema.extend( '$root', { allowAttributes: '$rootEditableOptions' } );
-
-		this.data.on( 'init', () => {
-			this.model.enqueueChange( { isUndoable: false }, writer => {
-				for ( const [ rootName, rootConfig ] of rootsConfig ) {
-					const root = this.model.document.getRoot( rootName )!;
-
-					// Set editable config for consistency with `addRoot()` method. This will allow features
-					// to use the same configuration for both initially loaded and dynamically added roots.
-					const rootEditableOptions: RootEditableOptions = {
-						...rootConfig.placeholder && { placeholder: rootConfig.placeholder },
-						...rootConfig.label && { label: rootConfig.label }
-					};
-
-					writer.setAttribute( '$rootEditableOptions', rootEditableOptions, root );
-				}
-			} );
-		} );
+		registerAndInitializeRootConfigAttributes( this );
 
 		const options = {
 			shouldToolbarGroupWhenFull: !this.config.get( 'toolbar.shouldNotGroupWhenFull' ),
@@ -447,7 +430,8 @@ export class MultiRootEditor extends Editor {
 
 	public addRoot( rootName: string, options: AddRootOptions & AddRootRootConfig = {} ): void {
 		const initialData: string = options.initialData || options.data || '';
-		const modelAttributes: EditorRootAttributes = options.modelAttributes || options.attributes || {};
+		const modelAttributes: EditorRootAttributes = { ...options.modelAttributes || options.attributes };
+		// eslint-disable-next-line ckeditor5-rules/no-literal-dollar-root -- public API default for `addRoot()`
 		const modelElement: string = options.modelElement || options.elementName || '$root';
 
 		if ( !this.model.schema.isLimit( modelElement ) ) {
@@ -473,6 +457,9 @@ export class MultiRootEditor extends Editor {
 			logWarning( 'multi-root-editor-add-root-element-option-ignored' );
 		}
 
+		// Storing editable options as a root attribute to make them available on other RTC clients.
+		ensureRootEditableOptions( modelAttributes, options );
+
 		const _addRoot = ( writer: ModelWriter ) => {
 			const root = writer.addRoot( rootName, modelElement );
 
@@ -484,14 +471,6 @@ export class MultiRootEditor extends Editor {
 				this.registerRootAttribute( key );
 				writer.setAttribute( key, modelAttributes[ key ], root );
 			}
-
-			// Storing editable options as a root attribute to make them available on other RTC clients.
-			const rootEditableOptions: RootEditableOptions = {
-				...options.placeholder && { placeholder: options.placeholder },
-				...options.label && { label: options.label }
-			};
-
-			writer.setAttribute( '$rootEditableOptions', rootEditableOptions, root );
 		};
 
 		if ( options.isUndoable ) {
@@ -1237,6 +1216,53 @@ function normalizeRootsAttributesConfig( config: Config<EditorConfig> ): void {
 	}
 }
 
+/**
+ * Normalize `placeholder` and `label` from `config.roots.<rootName>` into the `$rootEditableOptions` root model attribute,
+ * stored under `config.roots.<rootName>.modelAttributes`. This way the attribute is registered, set on initial data load
+ * and shipped through RTC initial-data path together with the rest of `modelAttributes`.
+ *
+ * This is also required by the revision history feature: on editor load, RH compares the latest revision data against
+ * `initialData` and `modelAttributes` passed to the editor and logs a warning if they do not match. Because `$rootEditableOptions`
+ * ends up in the revision data, it must also be present in `modelAttributes` (even as an empty object when no options
+ * are configured), otherwise the comparison reports a spurious mismatch.
+ */
+function normalizeRootEditableOptionsConfig( config: Config<EditorConfig> ): void {
+	const rootsConfig = config.get( 'roots' )!;
+
+	for ( const [ rootName, rootConfig ] of Object.entries( rootsConfig ) ) {
+		const modelAttributes: EditorRootAttributes = { ...rootConfig.modelAttributes };
+
+		if ( !ensureRootEditableOptions( modelAttributes, rootConfig ) ) {
+			continue;
+		}
+
+		config.set( `roots.${ rootName }.modelAttributes`, modelAttributes );
+	}
+}
+
+/**
+ * Mutates the given `modelAttributes` map by adding the `$rootEditableOptions` entry derived from `placeholder` and `label`.
+ * If `$rootEditableOptions` is already present, the map is left untouched.
+ *
+ * Returns `true` when the map was modified, `false` otherwise — useful to skip downstream work (e.g. `config.set`)
+ * when nothing changed.
+ */
+function ensureRootEditableOptions(
+	modelAttributes: EditorRootAttributes,
+	{ placeholder, label }: RootEditableOptions
+): boolean {
+	if ( '$rootEditableOptions' in modelAttributes ) {
+		return false;
+	}
+
+	modelAttributes.$rootEditableOptions = {
+		...placeholder && { placeholder },
+		...label && { label }
+	} satisfies RootEditableOptions;
+
+	return true;
+}
+
 function isElement( value: any ): value is Element {
 	return _isElement( value );
 }