The TBM_SETLINESIZE message sets the number of logical positions the trackbar’s slider moves in response to keyboard input from the arrow keys, such as the RIGHT ARROW or DOWN ARROW keys. The logical positions are the integer increments in the trackbar's range of minimum to maximum slider positions.
TBM_SETLINESIZE
wParam = 0; lParam = (LONG) lLineSize;
Returns a 32-bit value that specifies the previous line size.
The default setting for the line size is 1.
The trackbar also sends the TB_LINEUP and TB_LINEDOWN notification messages to its parent window when the user presses the arrow keys.