uses
  ComCtrls;

type
  TTrackBar = class(ComCtrls.TTrackBar)
  private
    procedure WMLBUTTONDOWN(var Msg: TMessage); message WM_LBUTTONDOWN;
  end;

// ...

implementation

{$R *.dfm}

// ...

uses
  CommCtrl;

procedure TTrackBar.WMLBUTTONDOWN(var Msg: TMessage);
var
  ChannelRect, SliderRect: TRect;
  SliderWidth: Word;
  Pt: TPoint;
begin
  ZeroMemory(@SliderRect, SizeOf(SliderRect));
  SendMessage(Handle, TBM_GETTHUMBRECT, 0, DWord(@SliderRect));
  ZeroMemory(@ChannelRect, SizeOf(ChannelRect));
  SendMessage(Handle, TBM_GETCHANNELRECT, 0, DWord(@ChannelRect));
  if Orientation = trHorizontal then
  begin
    Pt:= Point(msg.LParamLo, msg.LParamHi);
    SliderWidth := (SliderRect.Right - SliderRect.Left) div 2;
    ChannelRect.Top := SliderRect.Top;
    ChannelRect.Bottom := SliderRect.Bottom;
  end else
  begin
    Pt:= Point(msg.LParamHi, msg.LParamLo);
    SliderWidth := (SliderRect.Bottom - SliderRect.Top) div 2;
    ChannelRect.Top := SliderRect.Left;
    ChannelRect.Bottom := SliderRect.Right;
  end;
  if PtInRect(SliderRect, Point(msg.LParamLo, msg.LParamHi)) then
    inherited
  else
    if Windows.PtInRect(ChannelRect, Pt) then
    begin
      inc(ChannelRect.Left, SliderWidth);
      dec(ChannelRect.Right, SliderWidth);
      self.Position := round((Pt.X - ChannelRect.Left) * self.Max /
                             (ChannelRect.Right - ChannelRect.Left));
    end;
  inherited;
end;