Workarround1:
this.radDropDownList1.PopupClosing += radComboDemo_PopupClosing;
void radComboDemo_PopupClosing(object sender, RadPopupClosingEventArgs args)
{
            Point relativeMousePositionInPopup = ((RadDropDownListElement)sender).ListElement.ElementTree.Control.PointToClient(Control.MousePosition);
            args.Cancel = ((RadDropDownListElement)sender).ListElement.VScrollBar.ControlBoundingRectangle.Contains(relativeMousePositionInPopup);            
}

Workaround2:
class MyDropDownList : RadDropDownList
{
    public MyDropDownList()
    {
        this.PopupClosing += MyDropDownList_PopupClosing;
    }
 
    void MyDropDownList_PopupClosing(object sender, RadPopupClosingEventArgs args)
    {
        Point relativeMousePositionInPopup = ((RadDropDownListElement)sender).ListElement.ElementTree.Control.PointToClient(Control.MousePosition);
        args.Cancel = ((RadDropDownListElement)sender).ListElement.VScrollBar.ControlBoundingRectangle.Contains(relativeMousePositionInPopup);
    }
 
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadDropDownList).FullName;
        }
    }
}