procedure FilterItems(const Text: string; ListBox: TListBox);
var
  I: Integer;
  Hide: Boolean;
  Item: TListBoxItem;
  Head: TListBoxGroupHeader;
begin
  Head := nil;
  Hide := True;
  ListBox.BeginUpdate;
  try
    for I := 0 to ListBox.Content.ControlsCount - 1 do
    begin
      if Text.IsEmpty then
        ListBox.ListItems[I].Visible := True
      else
      if ListBox.ListItems[I] is TListBoxGroupHeader then
      begin
        if Assigned(Head) then
          Head.Visible := not Hide;
        Hide := True;
        Head := TListBoxGroupHeader(ListBox.ListItems[I]);
      end
      else
      if ListBox.ListItems[I] is TListBoxItem then
      begin
        Item := TListBoxItem(ListBox.ListItems[I]);
        if Item.Text.ToLower.Contains(Text) then
        begin
          Hide := False;
          Item.Visible := True;
        end
        else
          Item.Visible := False;
      end;
    end;
  finally
    ListBox.EndUpdate;
  end;
end;