winform 绘制无边框拉伸功能，被覆盖边界也可拉伸

#region 控制无边框拉伸
       private ResizeDirect _direct;
       private bool _resizeing;
       private Point _resizeStart;
       private Point _resizePos;
       private Size _resizeSize;

       const int WM_LBUTTONDOWN = 513;
       const int WM_MOUSEMOVE = 512;

       public bool PreFilterMessage(ref Message m)
       {
           if (IsDisposed)
           {
               Application.RemoveMessageFilter(this);
               return false;
           }

           var ctrl = FromChildHandle(m.HWnd);

           if (ctrl == null || ctrl.FindForm() != this)
           {
               return false;
           }

           //鼠标单击
           if (m.Msg == (int)WM_LBUTTONDOWN && _direct != ResizeDirect.None)
           {
               _resizeing = true;
               _resizeStart = MousePosition;
               _resizePos = Location;
               _resizeSize = Size;
               ResizeTask();
               return true;
           }

           //鼠标移动
           if (m.Msg == (int)WM_MOUSEMOVE)
           {
               if (_resizeing)
               {
                   switch (_direct)
                   {
                       case ResizeDirect.Bottom:
                       case ResizeDirect.Top:
                           Cursor = Cursors.SizeNS;
                           break;
                       case ResizeDirect.Left:
                       case ResizeDirect.Right:
                           Cursor = Cursors.SizeWE;
                           break;
                       case ResizeDirect.LeftTop:
                       case ResizeDirect.RightBottom:
                           Cursor = Cursors.SizeNWSE;
                           break;
                       case ResizeDirect.RightTop:
                       case ResizeDirect.LeftBottom:
                           Cursor = Cursors.SizeNESW;
                           break;
                   }
                   return true;
               }
               if (NoneBorderResize && FormBorderStyle == FormBorderStyle.None &&
                   WindowState == FormWindowState.Normal)
               {
                   _direct = ShowResizeCursor(PointToClient(MousePosition));
                   return _direct != ResizeDirect.None;
               }
           }

           return false;
       }

       private enum ResizeDirect
       {
           None,
           Left,
           Right,
           Top,
           Bottom,
           LeftTop,
           LeftBottom,
           RightTop,
           RightBottom
       }

       public void SafeInvoke(Action<object> callback, object state = null)
       {
           miniblinkBrowser1.SafeInvoke(callback, state);
       }

       private void ResizeTask()
       {
           var last = _resizeStart;
           var waiter = new SpinWait();

           Task.Factory.StartNew(() =>
           {
               while (_resizeing)
               {
                   if (MouseButtons != MouseButtons.Left)
                   {
                       _resizeing = false;
                       SafeInvoke(s => { Cursor = DefaultCursor; });
                       break;
                   }

                   var curr = MousePosition;
                   if (curr.X != last.X || curr.Y != last.Y)
                   {
                       var xx = curr.X - _resizeStart.X;
                       var xy = curr.Y - _resizeStart.Y;
                       int nx = Left, ny = Top, nw = Width, nh = Height;

                       switch (_direct)
                       {
                           case ResizeDirect.Left:
                               nw = _resizeSize.Width - xx;
                               nx = _resizePos.X + xx;
                               break;
                           case ResizeDirect.Right:
                               nw = _resizeSize.Width + xx;
                               break;
                           case ResizeDirect.Top:
                               nh = _resizeSize.Height - xy;
                               ny = _resizePos.Y + xy;
                               break;
                           case ResizeDirect.Bottom:
                               nh = _resizeSize.Height + xy;
                               break;
                           case ResizeDirect.LeftTop:
                               nw = _resizeSize.Width - xx;
                               nx = _resizePos.X + xx;
                               nh = _resizeSize.Height - xy;
                               ny = _resizePos.Y + xy;
                               break;
                           case ResizeDirect.LeftBottom:
                               nw = _resizeSize.Width - xx;
                               nx = _resizePos.X + xx;
                               nh = _resizeSize.Height + xy;
                               break;
                           case ResizeDirect.RightTop:
                               nw = _resizeSize.Width + xx;
                               nh = _resizeSize.Height - xy;
                               ny = _resizePos.Y + xy;
                               break;
                           case ResizeDirect.RightBottom:
                               nw = _resizeSize.Width + xx;
                               nh = _resizeSize.Height + xy;
                               break;
                       }
                       SafeInvoke(s =>
                       {
                           Size = new Size(nw, nh);
                           Location = new Point(nx, ny);
                       });
                   }
                   last = curr;
                   waiter.SpinOnce();
               }
           });
       }

       private ResizeDirect ShowResizeCursor(Point point)
       {
           var rect = ClientRectangle;
           var direct = ResizeDirect.None;

           if (point.X <= ResizeWidth.Left)
           {
               if (point.Y <= ResizeWidth.Top)
               {
                   direct = ResizeDirect.LeftTop;
               }
               else if (point.Y + ResizeWidth.Right >= rect.Height)
               {
                   direct = ResizeDirect.LeftBottom;
               }
               else
               {
                   direct = ResizeDirect.Left;
               }
           }
           else if (point.Y <= ResizeWidth.Top)
           {
               if (point.X <= ResizeWidth.Left)
               {
                   direct = ResizeDirect.LeftTop;
               }
               else if (point.X + ResizeWidth.Right >= rect.Width)
               {
                   direct = ResizeDirect.RightTop;
               }
               else
               {
                   direct = ResizeDirect.Top;
               }
           }
           else if (point.X + ResizeWidth.Right >= rect.Width)
           {
               if (point.Y <= ResizeWidth.Top)
               {
                   direct = ResizeDirect.RightTop;
               }
               else if (point.Y + ResizeWidth.Bottom >= rect.Height)
               {
                   direct = ResizeDirect.RightBottom;
               }
               else
               {
                   direct = ResizeDirect.Right;
               }
           }
           else if (point.Y + ResizeWidth.Bottom >= rect.Height)
           {
               if (point.X <= ResizeWidth.Left)
               {
                   direct = ResizeDirect.LeftBottom;
               }
               else if (point.X + ResizeWidth.Right >= rect.Width)
               {
                   direct = ResizeDirect.RightBottom;
               }
               else
               {
                   direct = ResizeDirect.Bottom;
               }
           }
           else if (IsMove == false)
           {
               if (Cursor != DefaultCursor)
               {
                   Cursor = DefaultCursor;
               }
           }

           switch (direct)
           {
               case ResizeDirect.Bottom:
               case ResizeDirect.Top:
                   Cursor = Cursors.SizeNS;
                   break;
               case ResizeDirect.Left:
               case ResizeDirect.Right:
                   Cursor = Cursors.SizeWE;
                   break;
               case ResizeDirect.LeftTop:
               case ResizeDirect.RightBottom:
                   Cursor = Cursors.SizeNWSE;
                   break;
               case ResizeDirect.RightTop:
               case ResizeDirect.LeftBottom:
                   Cursor = Cursors.SizeNESW;
                   break;
           }

           return direct;
       }

       #endregion