int GetHorizontalExtent() const;
Retrieves from a list box the width in pixels by which the list box can be scrolled horizontally if the list box has horizontal scroll bars.
To respond to GetHorizontalExtent, the list box must have been defined with the WS_HSCROLL style.
The scrollable width of the list box, in pixels.
CListBox::SetHorizontalExtent, LB_GETHORIZONTALEXTENT