public RadForm1()
        {
            InitializeComponent();

             DataTable dt = new DataTable();
            for (int i = 0; i < 20; i++)
            {
                dt.Columns.Add("col" + i, typeof(string));
            }
            for (int i = 0; i < 10; i++)
            {
                DataRow r = dt.NewRow();
                foreach (DataColumn col in dt.Columns)
                {
                    r[col.ColumnName] = Guid.NewGuid().ToString();
                }
                dt.Rows.Add(r);
            }
            this.radFilterView1.DataSource = dt;
        }

        protected override void OnShown(EventArgs e)
        {
            base.OnShown(e); 
            MethodInfo mi = typeof(RadFilterViewElement).GetMethod("UpdateScrollbars", BindingFlags.Instance | BindingFlags.NonPublic);
            mi.Invoke(this.radFilterView1.FilterViewElement, new object[] { this.radFilterView1.FilterViewElement.ElementsPanel.Size });
        }

        private void radFilterView1_CategoryCreated(object sender, Telerik.WinControls.UI.FilterView.FilterViewCategoryCreatedEventArgs e)
        {
            e.Category.ExpandedChanged += Category_ExpandedChanged;
            e.Category.Expanded = false;

        }

        private void Category_ExpandedChanged(object? sender, EventArgs e)
        {
            // You need pass the control to Windows main loop to perform the pending telerik events after property Expanded change in order to resize internally
            // the StackLayoutPanel Containers before calling UpdateScrollBars
            Application.DoEvents();
            // Now your code
            MethodInfo mi = typeof(RadFilterViewElement).GetMethod("UpdateScrollbars", BindingFlags.Instance | BindingFlags.NonPublic);
            mi.Invoke(this.radFilterView1.FilterViewElement, new object[] { this.radFilterView1.FilterViewElement.ElementsPanel.Size });
            // Hide the horizontal scrolll bar
            radFilterView1.FilterViewElement.HorizontalScrollBar.Visibility =ElementVisibility.Collapsed;
        }