Editor with a bottom toolbar and button grouping

import {
    DecoupledEditor,
    Plugin,
    Alignment,
    Autoformat,
    Bold,
    Italic,
    Strikethrough,
    Subscript,
    Superscript,
    Underline,
    BlockQuote,
    clickOutsideHandler,
    Essentials,
    Font,
    Heading,
    HorizontalLine,
    Image,
    ImageCaption,
    ImageResize,
    ImageStyle,
    ImageToolbar,
    ImageUpload,
    Indent,
    Link,
    List,
    MediaEmbed,
    Paragraph,
    RemoveFormat,
    Table,
    TableToolbar,
    DropdownButtonView,
    DropdownPanelView,
    DropdownView,
    ToolbarView
} from 'ckeditor5';
import { EasyImage } from 'ckeditor5-premium-features';
import fontColorIcon from '@ckeditor/ckeditor5-font/theme/icons/font-color.svg';

class FormattingOptions extends Plugin {
    /**
    * @inheritDoc
    */
    static get pluginName() {
        return 'FormattingOptions';
    }

    /**
    * @inheritDoc
    */
    constructor( editor ) {
        super( editor );

        editor.ui.componentFactory.add( 'formattingOptions', locale => {
            const t = locale.t;
            const buttonView = new DropdownButtonView( locale );
            const panelView = new DropdownPanelView( locale );
            const dropdownView = new DropdownView( locale, buttonView, panelView );
            const toolbarView = this.toolbarView = dropdownView.toolbarView = new ToolbarView( locale );

            // Accessibility: Give the toolbar a human-readable ARIA label.
            toolbarView.set( {
                ariaLabel: t( 'Formatting options toolbar' )
            } );

            // Accessibility: Give the dropdown a human-readable ARIA label.
            dropdownView.set( {
                label: t( 'Formatting options' )
            } );

            // Toolbars in dropdowns need specific styling, hence the class.
            dropdownView.extendTemplate( {
                attributes: {
                    class: [ 'ck-toolbar-dropdown' ]
                }
            } );

            // Accessibility: If the dropdown panel is already open, the arrow down key should focus the first child of the #panelView.
            dropdownView.keystrokes.set( 'arrowdown', ( data, cancel ) => {
                if ( dropdownView.isOpen ) {
                    toolbarView.focus();
                    cancel();
                }
            } );

            // Accessibility: If the dropdown panel is already open, the arrow up key should focus the last child of the #panelView.
            dropdownView.keystrokes.set( 'arrowup', ( data, cancel ) => {
                if ( dropdownView.isOpen ) {
                    toolbarView.focusLast();
                    cancel();
                }
            } );

            // The formatting options should not close when the user clicked:
            // * the dropdown or it contents,
            // * any editing root,
            // * any floating UI in the "body" collection
            // It should close, for instance, when another (main) toolbar button was pressed, though.
            dropdownView.on( 'render', () => {
                clickOutsideHandler( {
                    emitter: dropdownView,
                    activator: () => dropdownView.isOpen,
                    callback: () => { dropdownView.isOpen = false; },
                    contextElements: [
                        dropdownView.element,
                        ...[ ...editor.ui.getEditableElementsNames() ].map( name => editor.ui.getEditableElement( name ) ),
                        document.querySelector( '.ck-body-wrapper' )
                    ]
                } );
            } );

            // The main button of the dropdown should be bound to the state of the dropdown.
            buttonView.bind( 'isOn' ).to( dropdownView, 'isOpen' );
            buttonView.bind( 'isEnabled' ).to( dropdownView );

            // Using the font color icon to visually represent the formatting.
            buttonView.set( {
                tooltip: t( 'Formatting options' ),
                icon: fontColorIcon
            } );

            dropdownView.panelView.children.add( toolbarView );

            toolbarView.fillFromConfig(
                editor.config.get( 'formattingOptions' ),
                editor.ui.componentFactory
            );

            return dropdownView;
        } );
    }
}

DecoupledEditor
    .create( document.querySelector( '#editor-content' ), {
        licenseKey: 'GPL', // Or '<YOUR_LICENSE_KEY>'.
        plugins: [
            Alignment,
            Autoformat,
            BlockQuote,
            Bold,
            EasyImage,
            Essentials,
            Font,
            Heading,
            HorizontalLine,
            Image,
            ImageCaption,
            ImageResize,
            ImageStyle,
            ImageToolbar,
            ImageUpload,
            Indent,
            Italic,
            Link,
            List,
            MediaEmbed,
            Paragraph,
            RemoveFormat,
            Strikethrough,
            Subscript,
            Superscript,
            Table,
            TableToolbar,
            Underline,

            FormattingOptions
        ],
        toolbar: [
            'undo',
            'redo',
            '|',
            'formattingOptions',
            '|',
            'link',
            'blockQuote',
            'uploadImage',
            'insertTable',
            'mediaEmbed',
            'horizontalLine',
            '|',
            {
                label: 'Lists',
                icon: false,
                items: [ 'bulletedList', 'numberedList', '|', 'outdent', 'indent' ]
            }
        ],
        // Configuration of the formatting dropdown.
        formattingOptions: [
            'undo',
            'redo',
            '|',
            'fontFamily',
            'fontSize',
            'fontColor',
            'fontBackgroundColor',
            '|',
            'bold',
            'italic',
            'underline',
            'strikethrough',
            '|',
            'alignment',
            '|',
            'bulletedList',
            'numberedList',
            '|',
            'outdent',
            'indent',
            '|',
            'removeFormat'
        ],

        image: {
            resizeUnit: 'px',
            toolbar: [
                'imageStyle:inline',
                'imageStyle:wrapText',
                'imageStyle:breakText',
                '|',
                'toggleImageCaption',
                'imageTextAlternative'
            ]
        },

        table: {
            contentToolbar: [
                'tableColumn',
                'tableRow',
                'mergeTableCells'
            ]
        },
        cloudServices: {
            // This editor configuration includes the Easy Image feature.
            // Provide correct configuration values to use it.
            tokenUrl: 'https://example.com/cs-token-endpoint',
            uploadUrl: 'https://your-organization-id.cke-cs.com/easyimage/upload/'
            // For other image upload methods see the guide - https://ckeditor.com/docs/ckeditor5/latest/features/images/image-upload/image-upload.html.
        },
    } )
    .then( editor => {
        window.editor = editor;

        const toolbarContainer = document.querySelector( '#editor-toolbar-container' );

        toolbarContainer.appendChild( editor.ui.view.toolbar.element );

        overrideDropdownPositionsToNorth( editor, editor.ui.view.toolbar );
        overrideDropdownPositionsToNorth( editor, editor.plugins.get( 'FormattingOptions' ).toolbarView );

        overrideTooltipPositions( editor.ui.view.toolbar );
        overrideTooltipPositions( editor.plugins.get( 'FormattingOptions' ).toolbarView );
    } )
    .catch( err => {
        console.error( err.stack );
    } );

/**
 * Force all toolbar dropdown panels to use northern positions rather than southern (editor default).
 * This will position them correctly relative to the toolbar at the bottom of the editing root.
 *
 * @private
 * @param {module:core/editor/editor~Editor} editor
 * @param {module:ui/toolbar/toolbarview~ToolbarView} toolbarView
 */
function overrideDropdownPositionsToNorth( editor, toolbarView ) {
    const {
        south, north, southEast, southWest, northEast, northWest,
        southMiddleEast, southMiddleWest, northMiddleEast, northMiddleWest
    } = DropdownView.defaultPanelPositions;

    let panelPositions;

    if ( editor.locale.uiLanguageDirection !== 'rtl' ) {
        panelPositions = [
            northEast, northWest, northMiddleEast, northMiddleWest, north,
            southEast, southWest, southMiddleEast, southMiddleWest, south
        ];
    } else {
        panelPositions = [
            northWest, northEast, northMiddleWest, northMiddleEast, north,
            southWest, southEast, southMiddleWest, southMiddleEast, south
        ];
    }

    for ( const item of toolbarView.items ) {
        if ( !( item instanceof DropdownView ) ) {
            continue;
        }

        item.on( 'change:isOpen', () => {
            if ( !item.isOpen ) {
                return;
            }

            item.panelView.position = DropdownView._getOptimalPosition( {
                element: item.panelView.element,
                target: item.buttonView.element,
                fitInViewport: true,
                positions: panelPositions
            } ).name;
        } );
    }
}

/**
 * Forces all toolbar items to display tooltips to the north.
 * This will position them correctly relative to the toolbar at the bottom of the editing root.
 *
 * @param {module:ui/toolbar/toolbarview~ToolbarView} toolbarView
 */
function overrideTooltipPositions( toolbarView ) {
    for ( const item of toolbarView.items ) {
        if ( item.buttonView ) {
            item.buttonView.tooltipPosition = 'n';
        } else if ( item.tooltipPosition ) {
            item.tooltipPosition = 'n';
        }
    }
}

<style>
    #editor {
        display: flex;
        flex-direction: column;
    }

    #editor-content {
        border-bottom-left-radius: 0;
        border-bottom-right-radius: 0;
    }

    #editor-content:not(.ck-focused) {
        border-color: var(--ck-color-base-border);
    }

    #editor-toolbar-container > .ck.ck-toolbar {
        border-top-width: 0;
        border-top-left-radius: 0;
        border-top-right-radius: 0;
    }

    .ck.ck-content {
        font-family: Helvetica, Arial, sans-serif;
        padding: 3em 2em 2em;
    }

    .ck.ck-content h2 {
        border: 0;
        font-size: 1.3em;
        padding-top: 0.2em;
        padding-bottom: 0.2em;
        margin-bottom: 0.4em;
    }

    .ck.ck-content .ck-horizontal-line.ck-widget {
        text-align: center;
    }

    .ck.ck-content .ck-horizontal-line.ck-widget hr {
        margin: 5px auto;
        width: 50px;
        height: 1px;
        display: inline-block;
    }
</style>

<div id="editor">
    <div id="editor-content">
            Editor content is inserted here.
    </div>
    <div id="editor-toolbar-container"></div>
</div>