Automatically sets the value

Fixed two bugs

1) Autoselect first element

2) Fast ArrorBtn click 

void MultiColumnComboBoxElement_ArrowButton_Click(object sender, EventArgs e)
        {
            if (IsPopupVisible)
            {
                isArrorClick = true;
            }
        }

        void MultiColumnComboBoxElement_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (isArrorClick)
            {
                var oldRow = (e as CurrentRowChangedEventArgs)?.OldRow;
                if (oldRow == null)
                {
                    SelectedIndex = -1;
                }
                else
                {
                    SelectedIndex = oldRow.Index;
                }
            }
            isArrorClick = false;
        }

Hello, Yaroslav, 

I am glad that you have found a suitable solution for your scenario.

However, I would like to provide some more information in order the community to benefit from it. When you open the popup by pressing the arrow button and then close it by clicking the arrow again, it is not expected to change the selection in RadMultiColumnComboBox. The behavior on my end is the same. You need to explicitly select a row from the popup grid if you want to change the selection. Alternatively, you can set the RadMultiColumnComboBox.SelectedIndex property.

If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik