DataGridVewTextBoxCell with Span Behaviour

public static void SetDoubleBuffered(Control control)
{
    // set instance non-public property with name "DoubleBuffered" to true
    typeof(Control).InvokeMember("DoubleBuffered",
        BindingFlags.SetProperty | BindingFlags.Instance | BindingFlags.NonPublic,
        null, control, new object[] { true }
    );
}

protected override void Paint(Graphics graphics, Rectangle clipBounds, Rectangle cellBounds, int rowIndex, DataGridViewElementStates cellState, object value, object formattedValue, string errorText, DataGridViewCellStyle cellStyle, DataGridViewAdvancedBorderStyle advancedBorderStyle, DataGridViewPaintParts paintParts)
        {
            if (_ownerCell != null && _ownerCell.DataGridView == null)
            {
                _ownerCell = null; //owner cell was removed.
            }

            if (DataGridView == null || (_ownerCell == null && _columnSpan == 1 && _rowSpan == 1))
            {
                base.Paint(graphics, clipBounds, cellBounds, rowIndex, cellState, value, formattedValue, errorText, cellStyle, advancedBorderStyle, paintParts);
                return;
            }

            var ownerCell = this;
            var columnIndex = ColumnIndex;
            var columnSpan = _columnSpan;
            var rowSpan = _rowSpan;
            if (_ownerCell != null)
            {
                ownerCell = _ownerCell;
                columnIndex = _ownerCell.ColumnIndex;
                rowIndex = _ownerCell.RowIndex;
                columnSpan = _ownerCell.ColumnSpan;
                rowSpan = _ownerCell.RowSpan;
                value = _ownerCell.GetValue(rowIndex);
                errorText = _ownerCell.GetErrorText(rowIndex);
                cellState = _ownerCell.State;
                cellStyle = _ownerCell.GetInheritedStyle(null, rowIndex, true);
                formattedValue = _ownerCell.GetFormattedValue(value, rowIndex, ref cellStyle, null, null, DataGridViewDataErrorContexts.Display);
            }

            if (CellsRegionContainsSelectedCell(columnIndex, rowIndex, columnSpan, rowSpan))
            {
                cellState |= DataGridViewElementStates.Selected;
            }

            // Store the old Clip Bound
            RectangleF oldBounds = graphics.ClipBounds;
            var cellBounds2 = DataGridViewCellExHelper.GetSpannedCellBoundsFromChildCellBounds(this, cellBounds, DataGridView.SingleVerticalBorderAdded(), DataGridView.SingleHorizontalBorderAdded());
            clipBounds = DataGridViewCellExHelper.GetSpannedCellClipBounds(ownerCell, cellBounds2, DataGridView.SingleVerticalBorderAdded(), DataGridView.SingleHorizontalBorderAdded());

            advancedBorderStyle = DataGridViewCellExHelper.AdjustCellBorderStyle(ownerCell);

            using (var g = this.DataGridView.CreateGraphics())
            {
                // Set the new Clip Bounds to the bounds which are calulcated for the row to span.
                g.SetClip(clipBounds);

                //Paint the content.
                ownerCell.NativePaint(g, clipBounds, cellBounds2, rowIndex, cellState, value, formattedValue, errorText, cellStyle, advancedBorderStyle, paintParts & ~DataGridViewPaintParts.Border);

                //Paint the borders.
                if ((paintParts & DataGridViewPaintParts.Border) != DataGridViewPaintParts.None)
                {
                    var leftTopCell = ownerCell;
                    var advancedBorderStyle2 = new DataGridViewAdvancedBorderStyle
                    {
                        Left = advancedBorderStyle.Left,
                        Top = advancedBorderStyle.Top,
                        Right = DataGridViewAdvancedCellBorderStyle.None,
                        Bottom = DataGridViewAdvancedCellBorderStyle.None
                    };
                    leftTopCell.PaintBorder(g, clipBounds, cellBounds2, cellStyle, advancedBorderStyle2);

                    var rightBottomCell = DataGridView[columnIndex + columnSpan - 1, rowIndex + rowSpan - 1] as DataGridViewSpannedTextBoxCell ?? this;
                    var advancedBorderStyle3 = new DataGridViewAdvancedBorderStyle
                    {
                        Left = DataGridViewAdvancedCellBorderStyle.None,
                        Top = DataGridViewAdvancedCellBorderStyle.None,
                        Right = advancedBorderStyle.Right,
                        Bottom = advancedBorderStyle.Bottom
                    };
                    rightBottomCell.PaintBorder(g, clipBounds, cellBounds2, cellStyle, advancedBorderStyle3);

                }
            }

            // Set the new Clip Bounds to the bounds which are calulcated for the row to span.
            graphics.SetClip(clipBounds);

            //Paint the content.
            ownerCell.NativePaint(graphics, clipBounds, cellBounds2, rowIndex, cellState, value, formattedValue, errorText, cellStyle, advancedBorderStyle, paintParts & ~DataGridViewPaintParts.Border);

            //Paint the borders.
            if ((paintParts & DataGridViewPaintParts.Border) != DataGridViewPaintParts.None)
            {
                var leftTopCell = ownerCell;
                var advancedBorderStyle2 = new DataGridViewAdvancedBorderStyle
                {
                    Left = advancedBorderStyle.Left,
                    Top = advancedBorderStyle.Top,
                    Right = DataGridViewAdvancedCellBorderStyle.None,
                    Bottom = DataGridViewAdvancedCellBorderStyle.None
                };
                leftTopCell.PaintBorder(graphics, clipBounds, cellBounds2, cellStyle, advancedBorderStyle2);

                var rightBottomCell = DataGridView[columnIndex + columnSpan - 1, rowIndex + rowSpan - 1] as DataGridViewSpannedTextBoxCell ?? this;
                var advancedBorderStyle3 = new DataGridViewAdvancedBorderStyle
                {
                    Left = DataGridViewAdvancedCellBorderStyle.None,
                    Top = DataGridViewAdvancedCellBorderStyle.None,
                    Right = advancedBorderStyle.Right,
                    Bottom = advancedBorderStyle.Bottom
                };
                rightBottomCell.PaintBorder(graphics, clipBounds, cellBounds2, cellStyle, advancedBorderStyle3);

            }

            // Restore old Bounds! Otherwise only the spanned row will be painted
            graphics.SetClip(oldBounds);
        }

public class DataGridViewTextBoxCellEx: DataGridViewTextBoxCell, ISpannedCell
{
    ......
        protected override Size GetPreferredSize(Graphics graphics, DataGridViewCellStyle cellStyle, int rowIndex, Size constraintSize)
        {
            var size = base.GetPreferredSize(graphics, cellStyle, rowIndex, constraintSize);
            if (OwnerCell != null) return size;
            var grid = DataGridView;
            var height = size.Height - Enumerable.Range(RowIndex + 1, RowSpan - 1)
                                           .Select(index => grid.Rows[index].Height)
                                           .Sum();
            return new Size(size.Width, height);
        }
    ......
}

if (singleVerticalBorderAdded)
    clipBounds.Width++;
if (singleHorizontalBorderAdded)
    clipBounds.Height++;

if (singleVerticalBorderAdded)
{
    clipBounds.Width += 2;
    clipBounds.X--;
}
if (singleHorizontalBorderAdded)
{
    clipBounds.Height += 2;
    clipBounds.Y--;
}

if (singleVerticalBorderAdded)
    spannedCellBounds.Width++;
if (singleHorizontalBorderAdded)
    spannedCellBounds.Height++;

if (singleVerticalBorderAdded)
{
    spannedCellBounds.Width ++;
    spannedCellBounds.X--;
}
if (singleHorizontalBorderAdded)
{
    spannedCellBounds.Height ++;
    spannedCellBounds.Y--;
}