I finally found this:
public class MyWidget extends Composite implements HasAllMouseHandlers, HasClickHandlers {
...
public HandlerRegistration addClickHandler(ClickHandler handler) {
return addDomHandler(handler, ClickEvent.getType());
}
public HandlerRegistration addMouseDownHandler(MouseDownHandler handler) {
return addDomHandler(handler, MouseDownEvent.getType());
}
public HandlerRegistration addMouseMoveHandler(MouseMoveHandler handler) {
return addDomHandler(handler, MouseMoveEvent.getType());
}
public HandlerRegistration addMouseOutHandler(MouseOutHandler handler) {
return addDomHandler(handler, MouseOutEvent.getType());
}
public HandlerRegistration addMouseOverHandler(MouseOverHandler handler) {
return addDomHandler(handler, MouseOverEvent.getType());
}
public HandlerRegistration addMouseUpHandler(MouseUpHandler handler) {
return addDomHandler(handler, MouseUpEvent.getType());
}
public HandlerRegistration addMouseWheelHandler(MouseWheelHandler handler) {
return addDomHandler(handler, MouseWheelEvent.getType());
}
}
This answer: Issue in making composite widget draggable solved my issue and I can use any sort of Composite widgets with dnd now