When your window is resized, Windows uses the WM_NCCALCSIZE message to determine where your window's client area lives inside your window rectangle.

There are two forms of the WM_NCCALCSIZE message. The simple form merely takes a window rectangle and returns a client rectangle. This is useful for resizing a window to have a desired client rectangle, taking menu wrapping into account. The AdjustWindowRectEx function cannot take menu wrapping into account because it doesn't know which menu you are using. (Notice that there is no HMENU or HWND parameter to AdjustWindowRectEx.)

void
SetWindowClientSize(HWND hwnd, int cx, int cy)
{
    HMENU hmenu = GetMenu(hwnd);
    RECT rcWindow = { 0, 0, cx, cy };

    /*
     *  First convert the client rectangle to a window rectangle the
     *  menu-wrap-agnostic way.
     */
    AdjustWindowRectEx(&rcWindow, GetWindowStyle(hwnd), hmenu != NULL,
                       GetWindowExStyle(hwnd));

    /*
     *  If there is a menu, then check how much wrapping occurs
     *  when we set a window to the width specified by AdjustWindowRect
     *  and an infinite amount of height.  An infinite height allows
     *  us to see every single menu wrap.
     */
    if (hmenu) {
        RECT rcTemp = rcWindow;
        rcTemp.bottom = 0x7FFF;     /* "Infinite" height */
        SendMessage(hwnd, WM_NCCALCSIZE, FALSE, (LPARAM)&rcTemp);

        /*
         *  Adjust our previous calculation to compensate for menu
         *  wrapping.
         */
        rcWindow.bottom += rcTemp.top;
    }

    SetWindowPos(hwnd, NULL, 0, 0, rcWindow.right - rcWindow.left,
                 rcWindow.bottom - rcWindow.top, SWP_NOMOVE | SWP_NOZORDER);

}

Exercise: Explain why we used 0x7FFF to represent infinite height.

Exercise: Explain the line rcWindow.bottom += rcTemp.top.