Scrollable Dialog in Pure Win32 API

void SD_Initialize (HWND hwnd)
{						
	s_prevx = 1;
	s_prevy = 1;
 
	GetClientRect (hwnd, &InitialRect);
 
	GetClientRect (GetDesktopWindow(), &DesktopRect);
 
	SIZE       DlgSz = { InitialRect.right  - InitialRect.left + 1, 
		                 InitialRect.bottom - InitialRect.top  + 1};
 
	const int Slop = 40;	// leave some of the desk top unused

	const SIZE DskSz = { DesktopRect.right  - DesktopRect.left + 1 - Slop,
		                 DesktopRect.bottom - DesktopRect.top  + 1 - Slop};
 
	SCROLLINFO si;
 
	si.cbSize = sizeof(SCROLLINFO);
	si.fMask  = SIF_PAGE | SIF_POS | SIF_RANGE;
	si.nPos   = si.nMin = 1;
	si.nMax   = DlgSz.cx;
	si.nPage  = DlgSz.cx;
				
	SetScrollInfo (hwnd, SB_HORZ, &si, FALSE);
 
	si.nMax   = DlgSz.cy;
	si.nPage  = DlgSz.cy;
 
	SetScrollInfo (hwnd, SB_VERT, &si, FALSE);
 
// now see if window needs scroll bars.

	const int YOvr    = GetSystemMetrics (SM_CYHSCROLL);	// height of horizontal scroll bar
	const int XOvr    = GetSystemMetrics (SM_CXVSCROLL);	// width of vertical scroll bar
	bool      HScroll = FALSE, VScroll = FALSE;
 
	// get required size of window based on client size
	
	AdjustWindowRect (&InitialRect, GetWindowLong (hwnd, GWL_STYLE), FALSE);
 
	DlgSz.cx = InitialRect.right  - InitialRect.left + 1;
	DlgSz.cy = InitialRect.bottom - InitialRect.top  + 1;
 
	if (DlgSz.cy > DskSz.cy)		// do we need a vertical scroll bar
	{
		VScroll = TRUE;				//  yes
		DlgSz.cy  = DskSz.cy;		//   limit vertical size to available desk top vertical
		DlgSz.cx += XOvr;			//    horizontal size increases by width of vertical scroll bar
	}
 
	if (DlgSz.cx > DskSz.cx)		// do we need a horizontal scroll bar
	{
		HScroll = TRUE;			 	//  yes
		DlgSz.cx  = DskSz.cx;		//   limit horizontal size to available desktop horizontal
		DlgSz.cy += YOvr;			//    increase vertical size by height of horizontal scroll bar
									//     we may now need a vertical scroll bar if we didn't before
		if (!VScroll && (DlgSz.cy > DskSz.cy))
		{							//      yes, we now need a vertical scroll bar
			VScroll = TRUE;			//       we do now need a vertical scroll bar
			DlgSz.cy  = DskSz.cy;	//        limit vertical size to available desk top vertical
									//         no need to incease cx size since already at max
		}
		else if (DlgSz.cy > DskSz.cy)
		{
			DlgSz.cy = DskSz.cy;	// make sure cy not too large since it might 
			                        //  have been increased above
		}
	}
 
	// Resize the window if scroll bars are needed,  this will cause a WM_SIZE msg which the
	// dialog procedure should send off to SD_OnSize for processing

	if (HScroll || VScroll)
	{
		SetWindowPos (hwnd,HWND_TOP,0,0,DlgSz.cx,DlgSz.cy,SWP_NOMOVE | SWP_NOREPOSITION | SWP_SHOWWINDOW);
	}
 
} // end SD_Initialize

static int s_prevx = 1;
static int s_prevy = 1;

const bool needToScroll =
    (si.nPos != si.nMin && si.nPos == maxScrollPos) ||
    (state == SIZE_MAXIMIZED);
 
if(needToScroll)
{
    SD_ScrollClient(hwnd, bar[i], si.nPos);
}
 
...