Scrolling Text with the WM_HSCROLL and WM_VSCROLL Messages

The scroll bar sends WM_HSCROLL and WM_VSCROLL messages to the window procedure whenever the user clicks the scroll bar or drags the scroll box. The low-order words of WM_VSCROLL and WM_HSCROLL each contain a notification message that indicates the direction and magnitude of the scrolling action.

When the WM_HSCROLL and WM_VSCROLL messages are processed, the scroll bar notification message is examined and the scrolling increment is calculated. After the increment is applied to the current scrolling position, the window is scrolled to the new position by using the ScrollWindowEx function, and the position of the scroll box is adjusted by using the SetScrollInfo function. For operating system versions earlier than 4.0, use the SetScrollPos function to adjust the scroll box position.

After a window is scrolled, part of its client area is made invalid. To ensure that the invalid region is updated, the UpdateWindow function is used to generate a WM_PAINT message.