XPLM_API void                 XPLMSetWindowPositioningMode(
                                   XPLMWindowID         inWindowID,    
                                   XPLMWindowPositioningMode inPositioningMode,    
                                   int                  inMonitorIndex);    

Sets the policy for how X-Plane will position your window.

Some positioning modes apply to a particular monitor. For those modes, you can pass a negative monitor index to position the window on the main X-Plane monitor (the screen with the X-Plane menu bar at the top). Or, if you have a specific monitor you want to position your window on, you can pass a real monitor index as received from, e.g., XPLMGetAllMonitorBoundsOS().

Only applies to modern windows. (Windows created using the deprecated XPLMCreateWindow(), or windows compiled against a pre-XPLM300 version of the SDK will always use xplm_WindowPositionFree.)