glutPostOverlayRedisplay marks the overlay of the current window as needing to be redisplayed.
Mark the overlay of current window as needing to be redisplayed. The next iteration through glutMainLoop, the window's overlay display callback (or simply the display callback if no overlay display callback is registered) will be called to redisplay the window's overlay plane. Multiple calls to glutPostOverlayRedisplay before the next display callback opportunity (or overlay display callback opportunity if one is registered) generate only a single redisplay. glutPostOverlayRedisplay may be called within a window's display or overlay display callback to re-mark that window for redisplay.
Logically, overlay damage notification for a window is treated as a glutPostOverlayRedisplay on the damaged window. Unlike damage reported by the window system, glutPostOverlayRedisplay will not set to true the overlay's damaged status (returned by glutLayerGet(GLUT_OVERLAY_DAMAGED).
Also, see glutPostRedisplay.