运行时改变控件的大小（点击后立刻ReleaseCapture，然后计算位移，最后发消息改变位置）——最有趣的是TPanel其实也有窗口标题，因此可发HTCAPTION消息

//光标在控件不同位置时的样式

// 由于拐角这点手动精确实在困难 所以用范围 范围+3 这样很容易就找到这一点了
procedure CtrlMouseMove(Ctrl: TWinControl; Shift: TShiftState;X, Y: Integer);
begin
   with Ctrl do
   begin
      if (X >= 0) and (X <= 3) then
      begin
        if (Y >= 0) and (Y <= 3)                then  Cursor := crSizeNWSE;
        if (Y > 3) and (Y < Height - 3)         then  Cursor := cRsizewe;
        if (Y >= Height - 3) and (Y <= Height)  then  Cursor := crSizeNESW;
      end
      else if (X > 3) and (X < Width - 3) then
      begin
        if (Y >= 0) and (Y <= 3)                then  Cursor := crSizeNS;
        if (Y > 3) and (Y < Height - 3)         then  Cursor := cRarrow;
        if (Y >= Height - 3) and (Y <= Width)   then  Cursor := crSizeNS;
      end
      else if (X >= Width - 3) and (X <= Width) then
      begin
        if (Y >= 0) and (Y <= 3)                then  Cursor := crSizeNESW;
        if (Y > 3) and (Y < Height - 3)         then  Cursor := cRsizewe;
        if (Y >= Height - 3) and (Y <= Width)   then  Cursor := crSizeNWSE;
      end;
   end;
end;

//改变控件的大小

procedure CtrlMouseDown(Ctrl: TWinControl; Button: TMouseButton;  Shift: TShiftState; X, Y: Integer);
var
  WParam: Integer;
begin
  with Ctrl do
  begin
    if (X >= 0) and (X <= 3) then
    begin
      if (Y >= 0) and (Y <= 3)                 then  WParam:=$F004;
      if (Y > 3)  and (Y < Height - 3)         then  WParam:=$F001;
      if (Y >= Height - 3) and (Y <= Height)   then  WParam:=$F007;
    end
    else if (X > 3) and (X < Width - 3) then
    begin
      if (Y >= 0) and (Y <= 3)                 then  WParam:=$F003;
      if (Y > 3) and (Y < Height - 3)          then  WParam:=$F012;
      if (Y >= Height - 3) and (Y <= Width)    then  WParam:=$F006;
    end
    else if (X >= Width - 3) and (X <= Width)  then
    begin
      if (Y >= 0) and (Y <= 3)                 then  WParam:=$F005;
      if (Y > 3) and (Y < Height - 3)          then  WParam:=$F002;
      if (Y >= Height - 3) and (Y <= Width)    then  WParam:=$F008;
    end;
    ReleaseCapture; // 不断调用API，冒充鼠标释放
    SendMessage(Handle,WM_SYSCOMMAND,WParam,0); // 发消息改变ctrl的位置，受启发，也可改成SendMessage(Handle,WM_NCLBUTTONDOWN,HTCAPTION,0);
  end;
end;

使用

procedure TForm1.Panel1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  CtrlMouseDown(Panel1, Button, Shift, X, Y);
end;

procedure TForm1.Panel1MouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  CtrlMouseMove(Panel1, Shift, X, Y);
end;

参考： http://www.cnblogs.com/xe2011/p/3426494.html

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这些 WParam:=$F004; 是什么意思呢？MSDN说明：

https://msdn.microsoft.com/en-us/library/windows/desktop/ms646360(v=vs.85).aspx

还是不懂.

另外，这个例子为啥右键不能拖动？不也是MouseDown吗？

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