[Lazarus] Strange visibility bug with TCustomGroupBox descendants

[Torsten Bonde Christiansen]

Hi list.

For some time i have had a strange bug in my program which essentially 
boils down to a
TRadioGroupBox being visible, even though it has explicit been set to 
false (in code).

At first i though this was due to some strange coding of mine, but i 
have managed to
recreate the issue with a short sample (attached).

It seems that all TCustomGroupBox descendants (at least TGroupBox, 
RadioGroup, CheckGroup)
suffer from the same problem. Even more strange is that in order to 
provoke the groupbox
to become visible when not wanted, it MUST include a Frame, a 
PageControl, a Panel and a BitButton.

This bug is only present with the GTK2 widgetset. I have tested with 
Lazarus 1.2.6 and
latest trunk: Lazarus 1.3 r47043 FPC 2.6.4 x86_64-linux-gtk 2

Can anyone else confirm this bug? And what could be the cause of this?

Kind regards,
Torsten Bonde Christiansen.
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program project1;

{$mode objfpc}{$H+}

uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Interfaces, // this includes the LCL widgetset
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, ExtCtrls,
  Buttons, StdCtrls, ComCtrls;

{$R *.res}

var
  F: TForm;
  Fr: TFrame;
  P: TPanel;
  CloseBtn: TBitBtn;
  PG: TPageControl;
  TS: TTabSheet;
  GB: TCustomGroupBox;

begin
  RequireDerivedFormResource := True;
  Application.Initialize;
  Application.CreateForm(TForm, F);

  P := TPanel.Create(F);
  P.Align := alBottom;
  P.Height := 48;
  P.Parent := F;

  CloseBtn := TBitBtn.Create(F);
  CloseBtn.Kind := bkClose;
  CloseBtn.Parent := P;

  Fr := TFrame.Create(F);
  PG := TPageControl.Create(Fr);
  PG.Align := alClient;
  PG.Parent := Fr;

  TS := PG.AddTabSheet;
  TS.Caption := 'tab1';

  GB := TRadioGroup.Create(TS);
  GB.Parent := TS;
  GB.Top := 50;
  GB.Visible := false;

  GB := TGroupBox.Create(TS);
  GB.Parent := TS;
  GB.Top := 150;
  GB.Visible := false;

  GB := TCheckGroup.Create(TS);
  GB.Parent := TS;
  GB.Top := 250;
  GB.Visible := false;

  TS := PG.AddTabSheet;
  TS.Caption := 'tab2';


  Fr.Align := alClient;
  Fr.Parent := F;

  Application.Run;
end.