An application sends the CB_SETHORIZONTALEXTENT message to set the width, in pixels, by which a list box can be scrolled horizontally (the scrollable width). If the width of the list box is smaller than this value, the horizontal scroll bar horizontally scrolls items in the list box. If the width of the list box is equal to or greater than this value, the horizontal scroll bar is hidden or, if the combo box has the CBS_DISABLENOSCROLL style, disabled.
CB_SETHORIZONTALEXTENT wParam = (WPARAM) cxExtent; // scrollable width of list box, in pixels lParam = 0; // not used, must be zero
No return value.