How to resize borderless forms?

Change form's padding to 3,3,3,3 to leave a space for the mouse to click and drag

//resize form
        protected override void WndProc(ref Message m)
        {
            const UInt32 WM_NCHITTEST = 0x0084;
            const UInt32 WM_MOUSEMOVE = 0x0200;

            const UInt32 HTLEFT = 10;
            const UInt32 HTRIGHT = 11;
            const UInt32 HTBOTTOMRIGHT = 17;
            const UInt32 HTBOTTOM = 15;
            const UInt32 HTBOTTOMLEFT = 16;
            const UInt32 HTTOP = 12;
            const UInt32 HTTOPLEFT = 13;
            const UInt32 HTTOPRIGHT = 14;

            const int RESIZE_HANDLE_SIZE = 10;
            bool handled = false;
            if (m.Msg == WM_NCHITTEST || m.Msg == WM_MOUSEMOVE)
            {
                Size formSize = this.Size;
                Point screenPoint = new Point(m.LParam.ToInt32());
                Point clientPoint = this.PointToClient(screenPoint);

                Dictionary<UInt32, Rectangle> boxes = new Dictionary<UInt32, Rectangle>() {
            {HTBOTTOMLEFT, new Rectangle(0, formSize.Height - RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE)},
            {HTBOTTOM, new Rectangle(RESIZE_HANDLE_SIZE, formSize.Height - RESIZE_HANDLE_SIZE, formSize.Width - 2*RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE)},
            {HTBOTTOMRIGHT, new Rectangle(formSize.Width - RESIZE_HANDLE_SIZE, formSize.Height - RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE)},
            {HTRIGHT, new Rectangle(formSize.Width - RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE, formSize.Height - 2*RESIZE_HANDLE_SIZE)},
            {HTTOPRIGHT, new Rectangle(formSize.Width - RESIZE_HANDLE_SIZE, 0, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE) },
            {HTTOP, new Rectangle(RESIZE_HANDLE_SIZE, 0, formSize.Width - 2*RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE) },
            {HTTOPLEFT, new Rectangle(0, 0, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE) },
            {HTLEFT, new Rectangle(0, RESIZE_HANDLE_SIZE, RESIZE_HANDLE_SIZE, formSize.Height - 2*RESIZE_HANDLE_SIZE) }
        };

                foreach (KeyValuePair<UInt32, Rectangle> hitBox in boxes)
                {
                    if (hitBox.Value.Contains(clientPoint))
                    {
                        m.Result = (IntPtr)hitBox.Key;
                        handled = true;
                        break;
                    }
                }
            }

            if (!handled)
                base.WndProc(ref m);
        }