test.js

window.addEventListener("load", () => {
  TXTextControl.addEventListener("ribbonTabsLoaded", () => {
     // Obtain "Paragraph" ribbon group element (get further ribbon group element ids
     // by using the browser developer tools)
     const ribbonGroupParagraph = document.getElementById("ribbonGroupParagraph");

     const observer = new MutationObserver(mutations => {
        for (mutation of mutations) {
           switch (mutation.type) {
              case "attributes":
                 switch (mutation.attributeName) {
                    case "class":
                       // The "class" attribute of an element has been changed. 
                       // Get the affected element and figure out which button 
                       // was pressed via the element id.
                       const button = mutation.target;
                       switch (button.id) {
                          case "ribbonTabHome_drpDnBtnBulletList":
                             // "Bulleted list" button.
                             // The "toggled" state can be determined via the CSS class 
                             // "ribbon-button-selected".
                             if (button.classList.contains("ribbon-button-selected")) {
                                console.info("'Bulleted list' button toggled.")
                             }
                             else {
                                console.info("'Bulleted list' button un-toggled.")
                             }
                             break;

                          case "ribbonTabHome_drpDnBtnNumberedList":
                             // "Numbered list" button.
                             // ...
                             break;

                          // Get further button ids by browsing the html structure 
                          // using the browser developer tools.
                       }
                       break;
                 }
                 break;
           }
        }
     });

     // Observer configuration (We want to observe changes in the whole sub-tree of the 
     // "Paragraph" group root element).
     const config = { attributes: true, subtree: true };

     // Pass in the target node (in this case the "Paragraph" group root element) and 
     // the observer options.
     observer.observe(ribbonGroupParagraph, config);
  });
});