Ensures that a particular item is visible in the list-box portion of the combo box.
Specifies the zero-based index of the list-box item.
Zero if successful, or CB_ERR if an error occurs.
The system scrolls the list box until either the item specified by nIndex appears at the top of the list box or the maximum scroll range has been reached.
// Set the first visible item in the combo box to be the middle item
m_pComboBox->SetTopIndex(m_pComboBox->GetCount() / 2);