Clicking on the combo-box's button opens a popup listing the combo-box's items, and clicking on one selects this item, as expected. Clicking on an item that's already selected will de-select it, as is with a selection list.
LibCXXW's combo-box also offers full access with a keyboard. Standard combo-boxes receive focus, and are tabbable. A combo-box's label field can be tabbed to. With the input focus on the the combo-box label field, pressing a key selects the first combo-box item that starts with the character. If more than one item matches, pressing another key selects the first combo-box item that starts with those two characters, and so on.
Enter selects the next combo-box item that starts with the typed characters, if there's more than one. Del removes the currently selected item, making it possible to select a different item by typing.
After tabbing the input focus to the combo-box's button, Enter opens the combo-box popup. Pressing Enter again closes it. With the combo-box popup open, Cursor-Down highlights the first item in the combo-box. At this point the Cursor-Down and Cursor-Up navigate the popup just like a selection list; with Enter selecting the highlighted item. Esc closes the popup without making a new selection.