glutPushWindow man page
glutPushWindow — Request to lower the current window to the bottom.
OpenGLUT - window
This function requests that the
be ``pushed'' to the back.
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.
There may not be an immediate effect to this function. Wait for the glutWindowStatusFunc() callback to tell you about whatever obscured/visible status your window achieves.
The z-order of top-level windows is ultimately managed by the windowing system. Therefore, a push or pop request by an OpenGLUT application may not necessarily succeed or take immediate effect.
Not applicable to offscreen windows.