This message 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.
wParam
This parameter is not used.
lParam
This parameter is not used.
The return value is the scrollable width of the list box, in pixels.
To respond to the LB_GETHORIZONTALEXTENT message, the list box must have been defined with the WS_HSCROLL style.