WINDOWPLACEMENT g_wpPrev = { sizeof(g_wpPrev) };

void OnLButtonUp(HWND hwnd, int x, int y, UINT keyFlags)

{

  DWORD dwStyle = GetWindowLong(hwnd, GWL_STYLE);

  if (dwStyle & WS_OVERLAPPEDWINDOW) {

    MONITORINFO mi = { sizeof(mi) };

    if (GetWindowPlacement(hwnd, &g_wpPrev) &&

        GetMonitorInfo(MonitorFromWindow(hwnd,

                       MONITOR_DEFAULTTOPRIMARY), &mi)) {

      SetWindowLong(hwnd, GWL_STYLE,

                    dwStyle & ~WS_OVERLAPPEDWINDOW);

      SetWindowPos(hwnd, HWND_TOP,

                   mi.rcMonitor.left, mi.rcMonitor.top,

                   mi.rcMonitor.right - mi.rcMonitor.left,

                   mi.rcMonitor.bottom - mi.rcMonitor.top,

                   SWP_NOOWNERZORDER | SWP_FRAMECHANGED);

    }

  } else {

    SetWindowLong(hwnd, GWL_STYLE,

                  dwStyle | WS_OVERLAPPEDWINDOW);

    SetWindowPlacement(hwnd, &g_wpPrev);

    SetWindowPos(hwnd, NULL, 0, 0, 0, 0,

                 SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER |

                 SWP_NOOWNERZORDER | SWP_FRAMECHANGED);

  }

}

// add to WndProc

    HANDLE_MSG(hwnd, WM_LBUTTONUP, OnLButtonUp);