An application sends an LB_GETHORIZONTALEXTENT message to retrieve from a list box the width, in pixels, by which the list box can be scrolled horizontally (the scrollable width) if the list box has a horizontal scroll bar.
LB_GETHORIZONTALEXTENT wParam = 0; // not used; must be zero lParam = 0; // not used; must be zero
This message has no parameters.
The return value is the scrollable width, in pixels, of the list box.
To respond to the LB_GETHORIZONTALEXTENT message, the list box must have been defined with the WS_HSCROLL style.