Hi, Yaroslav,

Indeed, this is an improved solution considering the case when you have vertical and horizontal scrollbars in the drop down:

        private void EditorControl_MouseUp(object sender, MouseEventArgs e)
        {
            if (this.radMultiColumnComboBox1.SelectedIndex != -1 && e.X < this.radMultiColumnComboBox1.Width - 
                this.radMultiColumnComboBox1.MultiColumnComboBoxElement.EditorControl.TableElement.VScrollBar.Size.Width)
            {
               this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ClosePopup();
            }
        }

Thus, if you click on the vertical scrollbar, the popup won't be closed.

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

Hello, Peter,  

Following the provided information and sample video, I was able to observe the same behavior on my end when the user selects an already selected row in the drop down. After further testing I have noticed that the drop down is not closed in the specified situation as of version 2020.2.512.40.

I have logged it in our feedback portal by creating a public thread on your behalf. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.

I have also updated your Telerik points.

Currently, the possible solution that I can suggest is to close the drop down programmatically: 

        this.radMultiColumnComboBox1.EditorControl.MouseUp+=EditorControl_MouseUp;

        private void EditorControl_MouseUp(object sender, MouseEventArgs e)
        {
            this.radMultiColumnComboBox1.MultiColumnComboBoxElement.ClosePopup();
        }

Regards,

Dess | Tech Support Engineer, Sr.
Progress Telerik