Scrolls a rectangle of bits horizontally and vertically.
BOOL ScrollDC( int dx, int dy, LPCRECT lpRectScroll, LPCRECT lpRectClip, CRgn* pRgnUpdate, LPRECT lpRectUpdate );
If lpRectUpdate is NULL, Windows does not compute the update rectangle. If both pRgnUpdate and lpRectUpdate are NULL, Windows does not compute the update region. If pRgnUpdate is not NULL, Windows assumes that it contains a valid pointer to the region uncovered by the scrolling process (defined by the ScrollDC member function). The update region returned in lpRectUpdate can be passed to CWnd::InvalidateRgn if required.
An application should use the ScrollWindow member function of class CWnd when it is necessary to scroll the entire client area of a window. Otherwise, it should use ScrollDC.