도킹 폼에 다음과 같은 프로시져가 있습니다.
procedure CMDockClient(var Message: TCMDockClient); message CM_DOCKCLIENT;

CM_DOCKCLIENT 메시지를 받으면 실행 하는 프로시져 같은데 다음과 같습니다.

procedure TDockableForm.CMDockClient(var Message: TCMDockClient);
var
  ARect: TRect;
  DockType: TAlign;
  Host: TForm;
  Pt: TPoint;
begin
  //Overriding this message allows the dock form to create host forms
  //depending on the mouse position when docking occurs. If we don't override
  //this message, the form will use VCL's default DockManager.

  //NOTE: the only time ManualDock can be safely called during a drag
  //operation is we override processing of CM_DOCKCLIENT.

  if Message.DockSource.Control is TDockableForm then
  begin

    //Find out how to dock (Using a TAlign as the result of ComputeDockingRect)
    Pt.x := Message.MousePos.x;
    Pt.y := Message.MousePos.y;
    DockType := ComputeDockingRect(ARect, Pt);

    //if we are over a dockable form docked to a panel in the
    //main window, manually dock the dragged form to the panel with
    //the correct orientation.
    if (HostDockSite is TPanel) then
    begin
      Message.DockSource.Control.ManualDock(HostDockSite, nil, DockType);
      Exit;
    end;

    //alClient => Create a TabDockHost and manually dock both forms to the PageControl
    //owned by the TabDockHost.
    if DockType = alClient then
    begin
      Host := TTabDockHost.Create(Application);
      Host.BoundsRect := Self.BoundsRect;
      Self.ManualDock(TTabDockHost(Host).PageControl1, nil, alClient);
      Message.DockSource.Control.ManualDock(TTabDockHost(Host).PageControl1, nil, alClient);
      Host.Visible := True;
    end
    //if DockType <> alClient, create the ConjoinDockHost and manually dock both
    //forms to it. Be sure to make dockable forms non-dockable when hosted by
    // ConjoinDockForm, since it is using the VCL default DockManager.
    else begin
      Host := TConjoinDockHost.Create(Application);
      Host.BoundsRect := Self.BoundsRect;
      Self.ManualDock(Host, nil, alNone);
      Self.DockSite := False;
      Message.DockSource.Control.ManualDock(Host, nil, DockType);
      TDockableForm(Message.DockSource.Control).DockSite := False;
      Host.Visible := True;
    end;
  end;
end;

코드가 이해가 잘 되지 않아 그대로 따라 했는데 안 되는군요.
CM_DOCKCLIENT 메시지가 언제 어디서 날라 오는지 정말 모르겠군요.

좋은 답변 부탁드릴께요.
그럼 수고하세요.