the display of the drop-down part of the multicolumncombobox is different from the RAD combo box. There is an extra line between the content of the combo box and the drop-down button in the office 2016 theme at least. This should not be there to be consistent with the other combo boxes.
The RadMultiColumnComboBox has a separate keyboard functionality executed when the watermark textbox (the search box) is focused and the dropdown is open. Basically, you can navigate through the rows of the gridview part and select them using the keyboard.
Currently, this functionality is private. Introduce an API that allows you to customize the keyboard navigation. Consider something similar to the KeyboardCommandProvider of RadGridView.
Add non-editable feature to MultiColumnComboBox ( like on ComboBox ).