WM_MDIMAXIMIZE
hwndMaximize = (HWND) wParam; /* handle of child to maximize */
The WM_MDIMAXIMIZE message causes a multiple document interface (MDI) client window to maximize an MDI child window. When a child window is maximized, Windows resizes it to make its client area fill the client window. Windows places the child window's System menu (sometimes referred to as the Control menu) in the frame's menu bar so that the user can restore or minimize the child window; Windows adds the title of the child window to the frame window's menu of child windows.
hwndMaximize
Value of wParam. Identifies the child window to maximize.
An application should return zero if it processes this message.
If an MDI client window receives any message that changes the activation of its child windows while the currently active MDI child window is maximized, Windows restores the currently active child window and maximizes the newly activated child window.