MuseScore 4 was recently released, moving toolkits from Qt Widgets to Qt Quick, and completely refreshed the UI design. It’s a fantastic example of a modern desktop application written in Qt using the open-source (GPL) license. I’d mention it in the same breath as the KDE Plasma Desktop, Kdenlive, or Krita. The lead designer posted an excellent video discussing the technical and design process to YouTube. Sources are available from GitHub for the code curious.