终于懂了：Delphi重定义消息结构随心所欲，只需要前4个字节是消息编号就行了，跟Windows消息虽然尽量保持一致，但其实相互没有特别大的关系。有了这个，就有了主动，带不带句柄完全看需要。

  TWMContextMenu = packed record
    Msg: Cardinal;
    hWnd: HWND;
    case Integer of
      0: (
        XPos: Smallint;
        YPos: Smallint);
      1: (
        Pos: TSmallPoint;
        Result: Longint);
  end;

  TWMDropFiles = packed record
    Msg: Cardinal;
    Drop: THANDLE;
    Unused: Longint;
    Result: Longint;
  end;

  TWMSizeClipboard = packed record
    Msg: Cardinal;
    Viewer: HWND;
    RC: THandle;
    Result: Longint;
  end;

  TWMSize = packed record
    Msg: Cardinal;
    SizeType: Longint; { SIZE_MAXIMIZED, SIZE_MINIMIZED, SIZE_RESTORED,
                         SIZE_MAXHIDE, SIZE_MAXSHOW }
    Width: Word;
    Height: Word;
    Result: Longint;
  end;

  TWMTimer = packed record
    Msg: Cardinal;
    TimerID: Longint;
    TimerProc: TFarProc;
    Result: Longint;
  end;

  TWMSystemError = packed record
    Msg: Cardinal;
    ErrSpec: Word;
    Unused: Longint;
    Result: Longint;
  end;