A window can be in front of or behind other windows, as determined by the z-order from front to back. Top-level OpenGLUT windows can be placed at the front or back of the z-order by means of the glutPopWindow() and glutPushWindow() API functions.
A z-order also applies to the subwindows of a top-level window. While the z-order of top-level windows can usually be adjusted by the user, subwindow z-order is controlled entirely by the application.
If this has any effect on your window's visibility, you should receive a glutWindowStatusFunc() callback and a glutDisplayFunc() callback.
Not applicable to offscreen windows.