This message sets the width in pixels by which a list box can be scrolled horizontally. If the size of the list box is smaller than this value, the horizontal scroll bar will horizontally scroll items in the list box. If the list box is as large or larger than this value, the horizontal scroll bar is disabled.
wParam
Specifies the number of pixels by which the list box can be scrolled.
lParam
This parameter is not used.
To respond to the LB_SETHORIZONTALEXTENT message, the list box must have been defined with the WS_HSCROLL style.