BQt is a Blender add-on that integrates PySide6 QtWidgets with Blender, allowing developers to create custom UIs for their add-ons using Qt. The system wraps Blender's native window in a Qt container, manages the Qt event loop alongside Blender's main loop, and provides widget management to ensure Qt widgets stay properly parented and visible relative to Blender windows. It handles platform-specific window management for Windows and macOS to maintain proper focus and activation behavior, including a native event filter that forwards WM_ACTIVATE events on Windows to prevent focus issues when multiple Blender windows are open. The architecture features a modular design with abstract base classes for platform-specific implementations, clean separation of concerns between widget management, application logic, and utilities, and follows Blender add-on conventions.