C# vs2017 winForm 未解之谜（一）关于重绘tableLayoutPanel单元格边框后的问题

1.用label填充tableLayoutPanel1单元格，label的属性设置为：

anchor:Top, Bottom, Left, Right;

Dock:Fill;

margin:1,1,1,1

Rowspan:2;//根据需要

TextAline：MiddleCenter；

其他属性默认

tableLayoutPanel1的属性默认

得到的结果为：

为了显示表格边框线，在tableLayoutPanel1_CellPaint事件中绘制单元格边框：

        private void tableLayoutPanel1_CellPaint(object sender, TableLayoutCellPaintEventArgs e)
        { //绘制单元格边框
            Pen cpen = new Pen(Color.Black);
            cpen.Width = 1F;
            Rectangle rectangle = new Rectangle(e.CellBounds.X, e.CellBounds.Y, e.CellBounds.X + this.Width, e.CellBounds.Y + this.Height);
            e.Graphics.DrawRectangle(cpen,rectangle);
        }

得到的结果为（tableLayoutPanel1的底边和右边边框线不显示，这是问题一）：

补救办法一，在tableLayoutPanel1的四个边框处绘制矩形：

        private void SplitContainer1_Panel1_Paint(object sender, PaintEventArgs e)
        {//绘制表格边框
            Pen tpen = new Pen(Color.Black);
            tpen.Width = 1F;
            int x = tableLayoutPanel1.Bounds.X;
            int y = tableLayoutPanel1.Bounds.Y;
            int width = tableLayoutPanel1.Width;
            int height = tableLayoutPanel1.Height;
            //绘制矩形
            Rectangle rectangle = new Rectangle(x, y, x + width, y + height);
            e.Graphics.DrawRectangle(tpen, rectangle);
        }

得到的结果为：tableLayoutPanel1右边框和下边框和单元格之间有缝隙，这是问题二（tableLayoutPanel1放在SplitContainer1_Panel1内）：

补救办法二，单独绘制tableLayoutPanel1的下边框和右边框：

        private void SplitContainer1_Panel1_Paint(object sender, PaintEventArgs e)
        {//绘制表格右、下边框
            Pen tpen = new Pen(Color.Black);
            tpen.Width = 1F;
            int x = tableLayoutPanel1.Bounds.X;
            int y = tableLayoutPanel1.Bounds.Y;
            int width = tableLayoutPanel1.Width;
            int height = tableLayoutPanel1.Height;
            //绘制矩形
            //Rectangle rectangle = new Rectangle(x, y, x + width, y + height);
            //e.Graphics.DrawRectangle(tpen, rectangle);
            //绘制底边
            e.Graphics.DrawLine(tpen, x, y + height, x+width, y + height);
            //绘制右边
            e.Graphics.DrawLine(tpen, x + width, y, x + width, y + height);
        }        

得到结果为：

求问题一和问题二出现的原因？