Bizarre problem with Trackbar & Combo box

I have a trackbar and a combobox on a form. In the onchange event of the trackbar I clear the combobox and add some items to it. However, this causes the marker on the trackbar to zoom up to the mouse when it is clicked on the trackbar. If I remove the clear it is ok (but I get a build up of items in the combobox). If I don't do the clear, but delete the individual items I get the same behaviour. As both the trackbar and the combobox are windows common controls I cant track them past the points where the messages are sent to the appropriate handles.

Can anyone help me here?

I am using Delphi 3.02 on NT 4 SP3.

The source for the .DPR, .DFM and .PAS files are below.

program Project3;

uses
  Forms,
  Unit3 in 'Unit3.pas' {Form1};

{$R *.RES}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

--------------------------------------------------------------

object Form1: TForm1
  Left = 244
  Top = 115
  Width = 783
  Height = 540
  Caption = 'Form1'
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Sans Serif'
  Font.Style = []
  PixelsPerInch = 96
  TextHeight = 13
  object TrackBar1: TTrackBar
    Left = 80
    Top = 24
    Width = 617
    Height = 41
    Orientation = trHorizontal
    PageSize = 1
    Frequency = 1
    Position = 0
    SelEnd = 0
    SelStart = 0
    TabOrder = 0
    TickMarks = tmBottomRight
    TickStyle = tsAuto
    OnChange = TrackBar1Change
  end
  object ComboBox1: TComboBox
    Left = 312
    Top = 80
    Width = 369
    Height = 21
    ItemHeight = 13
    TabOrder = 1
    Text = 'ComboBox1'
  end
end

---------------------------------------------------------

unit Unit3;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
Dialogs,
  StdCtrls, ComCtrls, Menus;

type
  TForm1 = class(TForm)
    TrackBar1: TTrackBar;
    ComboBox1: TComboBox;
    procedure TrackBar1Change(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
with combobox1 do
begin
clear;
Items.Add('Start super');          {0}
Items.Add('Start spiral');         {1}
Items.Add('Start equal');          {2}

Items.Add('Start arc');            {3}
Items.Add('Start max');            {4}
Items.Add('End max');              {5}
Items.Add('End arc');              {6}
Items.Add('End equal');            {7}
Items.Add('End spiral');           {8}
Items.Add('End super');            {9}
Items.Add('User super');           {10}
end;
end;

end.