private void GvMrgLtr_CellDoubleClick(object sender, EventArgs e)
        {
            var m = MethodBase.GetCurrentMethod();
            try
            {
                if (sender is HostedTextBoxBase sndr)
                    sndr.Text = MemoDialog.ShowDialog(sndr.Text, "Edit", 0, false, false, true);
            }
            catch (Exception ex)
            {
                MessageBox.Show($@"{m.Name} : {ex}");
                Log.Error(ex, m.Name);
            }
        }

using System.Windows.Forms;
using Telerik.WinControls.UI;
using Telerik.WinForms.Controls.SyntaxEditor.Taggers;
using Telerik.WinForms.SyntaxEditor.Core.Text;

namespace P3CMS.Components
{
    internal static class MemoDialog
    {
        public static string ShowDialog(string text, string caption, int maxLength = 0,
            bool ro = false, bool spellcheck = true, bool sqlSyntaxHighlight = false)
        {
            var originalText = text;

            var memoDialogForm = new RadForm
            {
                Width = 500,
                Height = 700,
                FormBorderStyle = FormBorderStyle.Sizable,
                Text = caption,
                StartPosition = FormStartPosition.CenterScreen
            };

            var textBox = new RadTextBox
            {
                Left = 50,
                Top = 40,
                Width = 400,
                Dock = DockStyle.Fill,
                Multiline = true,
                AcceptsReturn = true,
                Text = text,
                ScrollBars = ScrollBars.Both,
                MaxLength = maxLength,
                ReadOnly = ro
            };

            var textDocument = new TextDocument(text);
            var radSyntaxEditor = new RadSyntaxEditor
            {
                Left = 50,
                Top = 40,
                Width = 400,
                Dock = DockStyle.Fill, 
                Document = textDocument
            };

            var tagger = new SqlTagger(radSyntaxEditor.SyntaxEditorElement);
            radSyntaxEditor.TaggersRegistry.RegisterTagger(tagger);

            var checker = new RadSpellChecker { AutoSpellCheckControl = textBox };
            if (!spellcheck)
                checker.AutoSpellCheckControl = null;

            var bottomPanel = new RadPanel { Height = 60, Dock = DockStyle.Bottom };
            var confirmation = new RadButton
            { Text = @"Ok", Left = 50, Width = 100, Top = 8, DialogResult = DialogResult.OK };
            var cancel = new RadButton
            { Text = @"Cancel", Left = 200, Width = 100, Top = 8, DialogResult = DialogResult.Cancel };

            confirmation.Click += (sender, e) => { memoDialogForm.Close(); };

            if (sqlSyntaxHighlight)
                memoDialogForm.Controls.Add(radSyntaxEditor);
            else
                memoDialogForm.Controls.Add(textBox);
            memoDialogForm.Controls.Add(bottomPanel);
            bottomPanel.Controls.Add(confirmation);
            bottomPanel.Controls.Add(cancel);
            memoDialogForm.AcceptButton = confirmation;
            memoDialogForm.CancelButton = cancel;

            if (!sqlSyntaxHighlight)
                textBox.Select(0, 0);

            return memoDialogForm.ShowDialog() == DialogResult.OK
                ? sqlSyntaxHighlight ? textDocument.CurrentSnapshot.GetText() : textBox.Text
                : originalText;
        }
    }
}

        private void MySyntaxEditor1_MouseDown(object sender, MouseEventArgs e)
        {  
            CaretPosition pos = mySyntaxEditor1.SyntaxEditorElement.GetPositionFromPoint(GetPosition(e, mySyntaxEditor1.SyntaxEditorElement.EditorPresenter)); 
            CaretPosition start = new CaretPosition(pos);
            start.MoveToCurrentWordStart();
            CaretPosition end = new CaretPosition(pos);
            end.MoveToCurrentWordEnd();
            mySyntaxEditor1.SyntaxEditorElement.Selection.Select(start, end);
        }

 public Telerik.WinControls.SyntaxEditor.UI.Point GetPosition(System.Windows.Forms.MouseEventArgs args, Telerik.WinControls.SyntaxEditor.UI.UIElement element)
        {

            System.Drawing.Point screenLocation = mySyntaxEditor1.SyntaxEditorElement.PointToScreen(args.Location);
            System.Drawing.Point point = element.PointFromScreen(screenLocation);
            Telerik.WinControls.Layouts.RadMatrix matrix = element.TotalTransform;
            matrix.Invert();

            return new System.Drawing.PointF(point.X * matrix.ScaleX, point.Y * matrix.ScaleY);
        }

mySyntaxEditor1.SyntaxEditorElement.HorizontalScrollBar.Padding = new Padding(0, 0, 0, 2);