From the GetScrollPos/GetScrollInfo/WM_VSCROLL/WM_HSCROLL references (in M$ Win32.hlp, missing from MSDN):
The limitation on this technique applies to real-time scrolling of a window's content. An application implements such scrolling by processing the WM_HSCROLL or WM_VSCROLL messages that carry the SB_THUMBTRACK notification message, thereby tracking the position of the scroll box, also known as the thumb, while the user moves it. Unfortunately, there is no function to retrieve the thumb's 32-bit position while the user moves it. GetScrollPos provides static position data only; an application can therefore only obtain 32-bit position data before or after a scroll has taken place.
The limitation on this technique applies to real-time scrolling of a window's content. An application implements such scrolling by processing the WM_HSCROLL or WM_VSCROLL messages that carry the SB_THUMBTRACK notification message, thereby tracking the position of the scroll box, also known as the thumb, while the user moves it. Unfortunately, there is no function to retrieve the thumb's 32-bit position while the user moves it. GetScrollPos provides static position data only; an application can therefore only obtain 32-bit position data before or after a scroll has taken place.