Win32 Plus Extra Height of Caption Bar

you set the size of the non-client area by handling the WM_NCCALCSIZE message. But don't do this unless you plan to do all of the non-client drawing as well by handling WM_NCPAINT

Edit: here are two code fragments, one that handles WM_NCCALCSIZE and provides a simple n pixel border, and another than adds some extra pixels after DefWindowProc has done the default handling.

case WM_NCCALCSIZE:
  {
  lRet = 0;
  const int cxBorder = 2;
  const int cyBorder = 2;
  InflateRect((LPRECT)lParam, -cxBorder, -cyBorder);
  }

case WM_NCCALCSIZE: 
  {
  LPNCCALCSIZE_PARAMS pncc = (LPNCCALCSIZE_PARAMS)lParam;
  //pncc->rgrc[0] is the new rectangle
  //pncc->rgrc[1] is the old rectangle
  //pncc->rgrc[2] is the client rectangle

  lRet = DefWindowProc (hwnd, WM_NCCALCSIZE, wParam, lParam);
  pncc->rgrc[0].top += ExtraCaptionHeight;
  }

You can learn a lot by passing WM_NCCALCSIZE to DefWindowProc and looking at the values of the NCCALCSIZEPARAM before and after.