RichTextEditor with an extra button

ModifiedRichTextEditor.java

package com.example.app.spring;

import com.vaadin.flow.component.ClientCallable;
import com.vaadin.flow.component.notification.Notification;
import com.vaadin.flow.component.richtexteditor.RichTextEditor;

public class ModifiedRichTextEditor extends RichTextEditor {

    public ModifiedRichTextEditor() {

        String script =
                "var span = document.createElement(\"span\");" +
                        "span.setAttribute(\"part\", \"toolbar-group\");" +
                        "var button = document.createElement(\"button\");" +
                        "button.textContent = \"hello\";" +
                        "button.setAttribute(\"part\", \"toolbar-button\");" +
                        "button.addEventListener('click', () => this.$server.clicked());" +
                        "span.appendChild(button);" +
                        "this.shadowRoot.querySelector('[part~=\"toolbar\"]').appendChild(span)";
        getElement().executeJs(script);
    }

    @ClientCallable
    private void clicked() {
        Notification.show("button clicked!");
    }

}