See https://wordpress.stackexchange.com/q/337978 for details.

my-block.css

/* See wp-includes/css/dist/block-library/editor.css */

.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-btn.mce-active button,
.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-btn.mce-active:hover button,
.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-btn.mce-active i,
.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-btn.mce-active:hover i {
  color: #23282d; }

.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-btn i {
  font-style: normal; }

.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-toolbar-grp > div {
  padding: 1px 3px; }

.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-list__block-edit::before {
  outline: 1px solid #e2e4e7; }

.editor-block-list__layout .editor-block-list__block[data-type="my-gutenberg/my-block"].is-hovered .editor-block-list__breadcrumb {
  display: none; }

div[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar + div {
  margin-top: 0;
  padding-top: 0; }

@media (min-width: 600px) {
  .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-switcher__no-switcher-icon {
    display: none; }
  .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar {
    float: right;
    margin-right: 23px;
    transform: translateY(-13px);
    top: 14px; }
    .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar .editor-block-toolbar {
      border: none;
      margin-top: 3px; } }
    @media (min-width: 600px) and (min-width: 782px) {
      .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar .editor-block-toolbar {
        margin-top: 0; } }

@media (min-width: 600px) {
      .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar .editor-block-toolbar::before {
        content: "";
        display: block;
        border-left: 1px solid #e2e4e7;
        margin-top: 4px;
        margin-bottom: 4px; }
    .editor-block-list__block[data-type="my-gutenberg/my-block"] .editor-block-contextual-toolbar .components-toolbar {
      background: transparent;
      border: none; }
  .editor-block-list__block[data-type="my-gutenberg/my-block"] .mce-container.mce-toolbar.mce-stack-layout-item {
    padding-right: 36px; } }

my-block.js

( function() {
	var el = wp.element.createElement;
	var ClassicEdit; // An instance of the private ClassicEdit class.

	wp.blocks.registerBlockType( 'my-gutenberg/my-block', {
		title: 'My Title',
		description: 'My cool block.',
		category: 'common',
		icon: 'email-alt',
		keywords: [ 'email' ],
		attributes: {
			content: {
				type: 'string',
				source: 'meta',
				meta: 'the_meta'
			}
		},
		supports: {
			customClassName: false,
		},
		edit: function( props ) {
			if ( ! ClassicEdit ) {
				var block = wp.blocks.getBlockType( 'core/freeform' );
				ClassicEdit = ( class extends block.edit {
					componentDidMount() {
						// Call the parent method to setup TinyMCE, etc.
						block.edit.prototype.componentDidMount.call( this );

						// Change the toolbar's title - to your block's.
						jQuery( '#toolbar-' + this.props.clientId )
							.attr( 'data-placeholder', 'My Title' );
					}
				} );
			}
			return el( ClassicEdit, props );
		},
		save: function( props ) {
		}
	} );
} )();

wpse-337978.php

<?php
/**
 * Plugin Name: WPSE 337978
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly.
}

// Load our block's assets.
add_action( 'enqueue_block_assets', function() {
	$plugin_url = plugin_dir_url( __FILE__ );
	wp_enqueue_style( 'my-block', $plugin_url . 'my-block.css', [ 'wp-edit-blocks' ], '' );
	wp_enqueue_script( 'my-block', $plugin_url . 'my-block.js', [ 'wp-editor' ], '', true );
} );

// Register the metadata for the REST API.
function gutenberg_my_block_init() {
	register_meta( 'post', 'the_meta', [
		'show_in_rest' => true,
		'single'       => true,
		'type'         => 'string',
	] );
}
add_action( 'init', 'gutenberg_my_block_init' );