From Jindra:

    F4
        Display/hide the items in the active list. Item that appears in the ComboBox is selected when the active list is open.
    Alt-Up/Down Arrow
        Same as above
    Enter (active list is open)
        Confirm the selection and close the active list
    Space (active list is open)
        Same as above
    Up/Down (active list is open)
        Move selection up/down one item
    Up/Down (non-editable ComboBox only)
        Change the items in the ComboBox