Hi Gadi,

I was not able to reproduce the described behavior. Below is the code snippet from my test project. I am also attaching a video showing the result on my end. Please open a support ticket and send us your project. A short video demonstrating the issue would also help.

 public partial class RadForm1 : Telerik.WinControls.UI.RadForm
 {
     public RadForm1()
     {
         InitializeComponent();

         this.radDropDownList1.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDown;
         
         this.radDropDownList1.AutoSizeItems = true;
         this.radDropDownList1.RightToLeft = RightToLeft.Yes;
         List<Item> items = new List<Item>();
         for (int i = 0; i < 150; i++)
         {
             items.Add(new Item(i, "Data" + i));
         }

         this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;
         this.radDropDownList1.DataSource = items;
         this.radDropDownList1.DisplayMember = "Description";
         this.radDropDownList1.ValueMember = "Id";
     }
 }

 public class Item
    {
        public int Id { get; set; }

        public string Description { get; set; }

        public Item(int id, string description)
        {
            this.Id = id;
            this.Description = description;
        }
    }


Regards,
Hristo