PoC of a single block editor.

tests/manual/single-block.html

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+<div id="editor">
+	This is an <b>editor</b>.
+</div>

tests/manual/single-block.js

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+/**
+ * @license Copyright (c) 2003-2024, CKSource Holding sp. z o.o. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
+ */
+
+/* globals console, window, document */
+
+import ClassicEditor from '@ckeditor/ckeditor5-editor-classic/src/classiceditor.js';
+import AutoImage from '@ckeditor/ckeditor5-image/src/autoimage.js';
+import AutoLink from '@ckeditor/ckeditor5-link/src/autolink.js';
+import Code from '@ckeditor/ckeditor5-basic-styles/src/code.js';
+import EasyImage from '@ckeditor/ckeditor5-easy-image/src/easyimage.js';
+import FindAndReplace from '@ckeditor/ckeditor5-find-and-replace/src/findandreplace.js';
+import FontBackgroundColor from '@ckeditor/ckeditor5-font/src/fontbackgroundcolor.js';
+import FontColor from '@ckeditor/ckeditor5-font/src/fontcolor.js';
+import FontFamily from '@ckeditor/ckeditor5-font/src/fontfamily.js';
+import FontSize from '@ckeditor/ckeditor5-font/src/fontsize.js';
+import Highlight from '@ckeditor/ckeditor5-highlight/src/highlight.js';
+import HtmlComment from '@ckeditor/ckeditor5-html-support/src/htmlcomment.js';
+import ImageResize from '@ckeditor/ckeditor5-image/src/imageresize.js';
+import ImageInsert from '@ckeditor/ckeditor5-image/src/imageinsert.js';
+import Mention from '@ckeditor/ckeditor5-mention/src/mention.js';
+import RemoveFormat from '@ckeditor/ckeditor5-remove-format/src/removeformat.js';
+import SourceEditing from '@ckeditor/ckeditor5-source-editing/src/sourceediting.js';
+import SpecialCharacters from '@ckeditor/ckeditor5-special-characters/src/specialcharacters.js';
+import SpecialCharactersEssentials from '@ckeditor/ckeditor5-special-characters/src/specialcharactersessentials.js';
+import Strikethrough from '@ckeditor/ckeditor5-basic-styles/src/strikethrough.js';
+import Subscript from '@ckeditor/ckeditor5-basic-styles/src/subscript.js';
+import Superscript from '@ckeditor/ckeditor5-basic-styles/src/superscript.js';
+import TextTransformation from '@ckeditor/ckeditor5-typing/src/texttransformation.js';
+import TextPartLanguage from '@ckeditor/ckeditor5-language/src/textpartlanguage.js';
+import Underline from '@ckeditor/ckeditor5-basic-styles/src/underline.js';
+import CloudServices from '@ckeditor/ckeditor5-cloud-services/src/cloudservices.js';
+import Style from '@ckeditor/ckeditor5-style/src/style.js';
+import GeneralHtmlSupport from '@ckeditor/ckeditor5-html-support/src/generalhtmlsupport.js';
+
+import { Essentials } from '@ckeditor/ckeditor5-essentials';
+import { Autoformat } from '@ckeditor/ckeditor5-autoformat';
+import { Bold, Italic } from '@ckeditor/ckeditor5-basic-styles';
+import { ImageInline, ImageStyle, ImageToolbar } from '@ckeditor/ckeditor5-image';
+import { Link } from '@ckeditor/ckeditor5-link';
+import { Paragraph } from '@ckeditor/ckeditor5-paragraph';
+
+import { CS_CONFIG } from '@ckeditor/ckeditor5-cloud-services/tests/_utils/cloud-services-config.js';
+
+ClassicEditor
+	.create( document.querySelector( '#editor' ), {
+		plugins: [
+			Essentials, Autoformat, Bold, ImageInline, ImageStyle, ImageToolbar, Italic, Link, Paragraph,
+			Underline, Strikethrough, Superscript, Subscript, Code, RemoveFormat,
+			FindAndReplace, FontColor, FontBackgroundColor, FontFamily, FontSize, Highlight,
+			EasyImage, ImageResize, ImageInsert, AutoImage, HtmlComment,
+			AutoLink, Mention, TextTransformation, SpecialCharacters, SpecialCharactersEssentials,
+			CloudServices, TextPartLanguage, SourceEditing, Style, GeneralHtmlSupport,
+			SingleBlock
+		],
+		toolbar: [
+			'style',
+			'|',
+			'removeFormat', 'bold', 'italic', 'strikethrough', 'underline', 'code', 'subscript', 'superscript', 'link',
+			'|',
+			'highlight', 'fontSize', 'fontFamily', 'fontColor', 'fontBackgroundColor',
+			'|',
+			'insertImage',
+			'specialCharacters',
+			'|',
+			'textPartLanguage',
+			'|',
+			'sourceEditing',
+			'|',
+			'undo', 'redo', 'findAndReplace'
+		],
+		cloudServices: CS_CONFIG,
+		image: {
+			styles: [
+				'alignCenter',
+				'alignLeft',
+				'alignRight'
+			],
+			resizeOptions: [
+				{
+					name: 'resizeImage:original',
+					label: 'Original size',
+					value: null
+				},
+				{
+					name: 'resizeImage:50',
+					label: '50%',
+					value: '50'
+				},
+				{
+					name: 'resizeImage:75',
+					label: '75%',
+					value: '75'
+				}
+			],
+			toolbar: [
+				'imageTextAlternative', '|', 'resizeImage'
+			]
+		},
+		placeholder: 'Type the content here!',
+		mention: {
+			feeds: [
+				{
+					marker: '@',
+					feed: [
+						'@apple', '@bears', '@brownie', '@cake', '@cake', '@candy', '@canes', '@chocolate', '@cookie', '@cotton', '@cream',
+						'@cupcake', '@danish', '@donut', '@dragée', '@fruitcake', '@gingerbread', '@gummi', '@ice', '@jelly-o',
+						'@liquorice', '@macaroon', '@marzipan', '@oat', '@pie', '@plum', '@pudding', '@sesame', '@snaps', '@soufflé',
+						'@sugar', '@sweet', '@topping', '@wafer'
+					],
+					minimumCharacters: 1
+				}
+			]
+		},
+		menuBar: {
+			isVisible: true
+		},
+		link: {
+			decorators: {
+				isExternal: {
+					mode: 'manual',
+					label: 'Open in a new tab',
+					attributes: {
+						target: '_blank',
+						rel: 'noopener noreferrer'
+					}
+				},
+				isDownloadable: {
+					mode: 'manual',
+					label: 'Downloadable',
+					attributes: {
+						download: 'download'
+					}
+				},
+				isGallery: {
+					mode: 'manual',
+					label: 'Gallery link',
+					classes: 'gallery'
+				}
+			}
+		}
+	} )
+	.then( editor => {
+		window.editor = editor;
+	} )
+	.catch( err => {
+		console.error( err.stack );
+	} );
+
+function SingleBlock( editor ) {
+	this.afterInit = function() {
+		const schema = editor.model.schema;
+
+		schema.extend( '$block', {
+			disallowIn: [ '$root', '$container' ]
+		} );
+		schema.extend( '$container', {
+			disallowIn: [ '$root', '$container' ]
+		} );
+		schema.extend( '$blockObject', {
+			disallowIn: [ '$root', '$container' ]
+		} );
+
+		schema.addAttributeCheck( context => {
+			return !context.endsWith( 'paragraph' );
+		} );
+
+		schema.extend( 'paragraph', {
+			allowIn: '$root',
+			isLimit: true,
+			isBlock: false
+		} );
+		schema.extend( '$text', {
+			allowIn: 'paragraph',
+			isInline: true,
+			isContent: true
+		} );
+		schema.extend( 'softBreak', {
+			disallowIn: 'paragraph'
+		} );
+
+		editor.conversion.for( 'dataDowncast' ).elementToElement( {
+			model: 'paragraph',
+			view: ( modelElement, { writer } ) => {
+				const viewElement = writer.createContainerElement( 'div' );
+
+				writer.setCustomProperty( 'dataPipeline:transparentRendering', true, viewElement );
+
+				return viewElement;
+			},
+			converterPriority: 'high'
+		} );
+
+		editor.conversion.for( 'editingDowncast' ).elementToElement( {
+			model: 'paragraph',
+			view: 'div',
+			converterPriority: 'high'
+		} );
+	};
+}