[Lazarus] Trackbar - how go to mouse click position?

[Krzysztof]

Ok, I read that on GTK, middle mouse button is responsible for jump to
position. So I am trying to simulate it:

procedure TForm1.TrackBar1Click(Sender: TObject);
begin
  PostMessage(TrackBar1.Handle, LM_MBUTTONDOWN, 0, 0);
  //TrackBar1.Perform(LM_MBUTTONDOWN,0,0);
end;

But it doesn't work. Here is some python solution but I don't understand it:
https://mail.gnome.org/archives/gtk-list/2011-November/msg00032.html