An application sends the CB_GETHORIZONTALEXTENT message to retrieve from a combo box the width, in pixels, by which the list box can be scrolled horizontally (the scrollable width). This is applicable only if the list box has a horizontal scroll bar.
CB_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.