XPUDragWidget
From X-Plane SDK
XPUDragWidget
WIDGET_API int XPUDragWidget( XPWidgetMessage inMessage, XPWidgetID inWidget, intptr_t inParam1, intptr_t inParam2, int inLeft, int inTop, int inRight, int inBottom);XPUDragWidget drags the widget in response to mouse clicks. Pass in not only the event, but the global coordinates of the drag region, which might be a sub-region of your widget (for example, a title bar).