DOC: Vertical Slider Control Sends WM_VSCROLL Message

A slider control notifies its parent window of user actions by sending the parent WM_HSCROLL messages. You can override the parent's OnHScroll member function to handle these messages. This function will be passed a notification code, the position of the slider, and a pointer to the CSliderCtrl object. Note that the pointer is of type CScrollBar* even though it points to a CSliderCtrl object. You may need to typecast this pointer if you need to manipulate the slider control."

This information is true only for a HORIZONTAL slider control (a trackbar with the TBS_HORZ style). For a VERTICAL slider control (a trackbar with the TBS_VERT style), the WM_VSCROLL will be sent instead of the WM_HSCROLL message. You should override the parent's OnVScroll() member function to handle these messages.