C# vs2017 winForm 未解之謎（一）關於重繪tableLayoutPanel單元格邊框后的問題

1.用label填充tableLayoutPanel1單元格，label的屬性設置為：

anchor:Top, Bottom, Left, Right;

Dock:Fill;

margin:1,1,1,1

Rowspan:2;//根據需要

TextAline：MiddleCenter；

其他屬性默認

tableLayoutPanel1的屬性默認

得到的結果為：

為了顯示表格邊框線，在tableLayoutPanel1_CellPaint事件中繪制單元格邊框：

        private void tableLayoutPanel1_CellPaint(object sender, TableLayoutCellPaintEventArgs e)
        { //繪制單元格邊框
            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);
        }

得到的結果為（tableLayoutPanel1的底邊和右邊邊框線不顯示，這是問題一）：

補救辦法一，在tableLayoutPanel1的四個邊框處繪制矩形：

        private void SplitContainer1_Panel1_Paint(object sender, PaintEventArgs e)
        {//繪制表格邊框
            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;
            //繪制矩形
            Rectangle rectangle = new Rectangle(x, y, x + width, y + height);
            e.Graphics.DrawRectangle(tpen, rectangle);
        }

得到的結果為：tableLayoutPanel1右邊框和下邊框和單元格之間有縫隙，這是問題二（tableLayoutPanel1放在SplitContainer1_Panel1內）：

 

補救辦法二，單獨繪制tableLayoutPanel1的下邊框和右邊框：

        private void SplitContainer1_Panel1_Paint(object sender, PaintEventArgs e)
        {//繪制表格右、下邊框
            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;
            //繪制矩形
            //Rectangle rectangle = new Rectangle(x, y, x + width, y + height);
            //e.Graphics.DrawRectangle(tpen, rectangle);
            //繪制底邊
            e.Graphics.DrawLine(tpen, x, y + height, x+width, y + height);
            //繪制右邊
            e.Graphics.DrawLine(tpen, x + width, y, x + width, y + height);
        }        

得到結果為：

求問題一和問題二出現的原因？