The TBM_SETPAGESIZE message sets the number of logical positions the trackbar’s slider moves in response to keyboard input, such as the PAGE UP or PAGE DOWN keys, or mouse input, such as clicks in the trackbar’s channel. The logical positions are the integer increments in the trackbar's range of minimum to maximum slider positions.
TBM_SETPAGESIZE
wParam = 0; lParam = (LONG) lPageSize;
Returns a 32-bit value that specifies the previous page size.
The trackbar also sends the TB_PAGEUP and TB_PAGEDOWN notification messages to its parent window when it receives keyboard or mouse input that scrolls the page.