typedef void (* XPLMReceiveMonitorBoundsOS_f)(
                         int                  inMonitorIndex,    
                         int                  inLeftPx,    
                         int                  inTopPx,    
                         int                  inRightPx,    
                         int                  inBottomPx,    
                         void *               inRefcon);    

This function is informed of the global bounds (in pixels) of a particular monitor within the operating system’s global desktop space. Note that a monitor index being passed to you here does not indicate that X-Plane is running in full screen on this monitor, or even that any X-Plane windows exist on this monitor.