MSDN: EN_VSCROLL This message is sent for the following mouse events on the vertical scroll bar: clicking either arrow button or clicking between the arrow button and the thumb. However, the message is not sent when clicking the scroll bar mouse itself.
EN_VSCROLL
This message is sent for the following mouse events on the vertical scroll bar: clicking either arrow button or clicking between the arrow button and the thumb. However, the message is not sent when clicking the scroll bar mouse itself.