TScrollBar.Visible turns False when Kind is changed between sbHorizontal and sbVertical at design-time

Original Reporter info from Mantis: russdirks

Reporter name: Russel Dirks

Description:

Summary and description edited by Juha to match the real issue.

Visible property changes automatically but only at design time and only with GTK2 widgetset.
Changing the orientation at run time works. For example:

procedure TForm1.Button1Click(Sender: TObject);
begin
  if ScrollBar1.Kind=sbHorizontal then
    ScrollBar1.Kind:=sbVertical
  else
    ScrollBar1.Kind:=sbHorizontal;
  Label1.Caption:='Visible=' + BoolToStr(ScrollBar1.Visible, True);
end;

Steps to reproduce:

Create a new GUI application
Drop a new TScrollBar on the form. It defaults to horizontal. Note: Visible=True.
Change orientation (Kind property) to vertical in OI. Suddenly Visible=False.

Additional information:

I've seen this in Lazarus 1.6RC1 and 1.6RC2, under Ubuntu 14.04 and 15.10.

Mantis conversion info:

Mantis ID: 29422
OS: Ubuntu
OS Build: 14.04 / 15.10
Build: 2015/12/07
Platform: x-86_64-linux-gtk 2
Version: 1.6RC1
Fixed in revision: r51361 (#871f6315)
Monitored by: » russdirks (Russel Dirks)