XPLM_API void       XPLMGetWindowGeometryOS(
                         XPLMWindowID         inWindowID,    
                         int *                outLeft,    /* Can be NULL */
                         int *                outTop,    /* Can be NULL */
                         int *                outRight,    /* Can be NULL */
                         int *                outBottom);    /* Can be NULL */

This routine returns the position and size of a “popped out” window (i.e., a window whose positioning mode is xplm_WindowPopOut), in operating system pixels. Pass NULL to not receive any parameter.