/// <summary>
        /// Calculates the size of the window frame and client area of the RichTextBox
        /// </summary>
        void WmNccalcsize(ref Message m)
        {
            // let the richtextbox control draw the scrollbar if necessary.
            base.WndProc(ref m);

            // we visual styles are not enabled and BorderStyle is not Fixed3D then we have nothing more to do.
            if (!this.RenderWithVisualStyles())
            {
                return;
            }

            // contains detailed information about WM_NCCALCSIZE message
            NativeMethods.NCCALCSIZE_PARAMS par = new NativeMethods.NCCALCSIZE_PARAMS();

            // contains the window frame RECT
            NativeMethods.RECT windowRect;

            if (m.WParam == IntPtr.Zero) // LParam points to a RECT struct
            {
                windowRect = (NativeMethods.RECT)Marshal.PtrToStructure(m.LParam, typeof(NativeMethods.RECT));
            }
            else // LParam points to a NCCALCSIZE_PARAMS struct
            {
                par        = (NativeMethods.NCCALCSIZE_PARAMS)Marshal.PtrToStructure(m.LParam, typeof(NativeMethods.NCCALCSIZE_PARAMS));
                windowRect = par.rgrc0;
            }

            // contains the client area of the control
            NativeMethods.RECT contentRect;

            // get the DC
            IntPtr hDC = NativeMethods.GetWindowDC(this.Handle);

            // open theme data
            IntPtr hTheme = NativeMethods.OpenThemeData(this.Handle, "EDIT");

            // find out how much space the borders needs
            if (NativeMethods.GetThemeBackgroundContentRect(hTheme, hDC, NativeMethods.EP_EDITTEXT, NativeMethods.ETS_NORMAL
                                                            , ref windowRect
                                                            , out contentRect) == NativeMethods.S_OK)
            {
                // shrink the client area the make more space for containing text.
                contentRect.Inflate(-1, -1);

                // remember the space of the borders
                this.borderRect = new NativeMethods.RECT(contentRect.Left - windowRect.Left
                                                         , contentRect.Top - windowRect.Top
                                                         , windowRect.Right - contentRect.Right
                                                         , windowRect.Bottom - contentRect.Bottom);

                // update LParam of the message with the new client area
                if (m.WParam == IntPtr.Zero)
                {
                    Marshal.StructureToPtr(contentRect, m.LParam, false);
                }
                else
                {
                    par.rgrc0 = contentRect;
                    Marshal.StructureToPtr(par, m.LParam, false);
                }

                // force the control to redraw it´s client area
                m.Result = new IntPtr(NativeMethods.WVR_REDRAW);
            }

            // release theme data handle
            NativeMethods.CloseThemeData(hTheme);

            // release DC
            NativeMethods.ReleaseDC(this.Handle, hDC);
        }
        /// <summary>
        /// The border painting is done here.
        /// </summary>
        void WmNcpaint(ref Message m)
        {
            base.WndProc(ref m);

            if (!this.RenderWithVisualStyles())
            {
                return;
            }

            // set the part id to TextBox
            int partId = NativeMethods.EP_EDITTEXT;

            // set the state id of the current TextBox
            int stateId;

            if (this.Enabled)
            {
                if (this.ReadOnly)
                {
                    stateId = NativeMethods.ETS_READONLY;
                }
                else
                {
                    stateId = NativeMethods.ETS_NORMAL;
                }
            }
            else
            {
                stateId = NativeMethods.ETS_DISABLED;
            }

            // define the windows frame rectangle of the TextBox
            NativeMethods.RECT windowRect;
            NativeMethods.GetWindowRect(this.Handle, out windowRect);
            windowRect.Right -= windowRect.Left; windowRect.Bottom -= windowRect.Top;
            windowRect.Top    = windowRect.Left = 0;

            // get the device context of the window frame
            IntPtr hDC = NativeMethods.GetWindowDC(this.Handle);

            // define a rectangle inside the borders and exclude it from the DC
            NativeMethods.RECT clientRect = windowRect;
            clientRect.Left   += this.borderRect.Left;
            clientRect.Top    += this.borderRect.Top;
            clientRect.Right  -= this.borderRect.Right;
            clientRect.Bottom -= this.borderRect.Bottom;
            NativeMethods.ExcludeClipRect(hDC, clientRect.Left, clientRect.Top, clientRect.Right, clientRect.Bottom);

            // open theme data
            IntPtr hTheme = NativeMethods.OpenThemeData(this.Handle, "EDIT");

            // make sure the background is updated when transparent background is used.
            if (NativeMethods.IsThemeBackgroundPartiallyTransparent(hTheme
                                                                    , NativeMethods.EP_EDITTEXT, NativeMethods.ETS_NORMAL) != 0)
            {
                NativeMethods.DrawThemeParentBackground(this.Handle, hDC, ref windowRect);
            }

            // draw background
            NativeMethods.DrawThemeBackground(hTheme, hDC, partId, stateId, ref windowRect, IntPtr.Zero);

            // close theme data
            NativeMethods.CloseThemeData(hTheme);

            // release dc
            NativeMethods.ReleaseDC(this.Handle, hDC);

            // we have processed the message so set the result to zero
            m.Result = IntPtr.Zero;
        }