Thank you for writing to us.
I have looked at the issue you described. Unfortunately at this stage it is not possible to edit the data displayed in a RadMultiColumnComboBox. This is by design and the control will internally close its drop-down on either CurrentRowChanged or CellClick event.
However we do agree that there is further space for improvements in this direction and we will consider implementing more fine-tunable drop-down behavior for this component.
I would like to thank you for your feedback which is of great importance to us.
All the best,
the Telerik team