freeglut_return_if_fail( fgStructure.Window->Callbacks.Visibility != NULL );
if( status == GLUT_HIDDEN || status == GLUT_FULLY_COVERED )
freeglut_return_if_fail( fgStructure.Window->Callbacks.Visibility != NULL );
if( status == GLUT_HIDDEN || status == GLUT_FULLY_COVERED )