unit untCustomWindow;

interface

uses
 Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
 TCustomWindow = class(TForm)
 private
   FAsChild : Boolean;
   FTempParent : TWinControl;
 protected
   procedure CreateParams (var Params : TCreateParams); override;
   procedure Loaded; override;
 public
   constructor Create (AOwner : TComponent;
                       AParent : TWinControl); reintroduce;
 end;

implementation

{$R *.dfm}

constructor TCustomWindow.Create (AOwner : TComponent; AParent : TWinControl);
begin
 FAsChild := true;
 FTempParent := AParent;
 inherited Create (AOwner);
end;

procedure TCustomWindow.CreateParams (var Params : TCreateParams);
begin
 inherited;
 if FAsChild then
   Params.Style := Params.Style or WS_CHILD;
end;

procedure TCustomWindow.Loaded;
begin
 inherited;
 if FAsChild then begin
   Align := alClient;
   BorderStyle := bsNone;
   BorderIcons := [];
   Parent := FTempParent;
   Position := poDefault;
 end;
end;

end.

TMyWindow.Create(self, MyTabSheet);