An application sends an LB_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.
LB_SETHORIZONTALEXTENT wParam = (WPARAM) cxExtent; // horizontal scroll width lParam = 0; // not used; must be zero
Windows 95: The wParam parameter is limited to 16-bit values.
This message does not return a value.
To respond to the LB_SETHORIZONTALEXTENT message, the list box must have been defined with the WS_HSCROLL style.