unit Hndltabu;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes,
  Graphics, Controls, Forms, Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Label1: TLabel;
    procedure FormCreate(Sender: TObject);
  private { Private-Deklarationen }
    procedure AppMessage(var Msg: TMsg; var Handled: Boolean);
  public { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.AppMessage(var Msg: TMsg; var Handled: Boolean);
const
  shiftPressed: boolean = false;
begin
  if Msg.Message = WM_KEYDOWN then
    if not shiftPressed and (Msg.wParam = VK_SHIFT) then
    begin
      shiftPressed := true;
      Exit;
    end
    else
    begin
      if Msg.wParam = VK_TAB then
        if ActiveControl = Edit1 then
        begin
          if shiftPressed then
            Label1.Caption := 'BACKTAB!'
          else
            Label1.Caption := 'TAB!';

          Handled := true
        end
        else
          Label1.Caption := '';

      shiftPressed := false;
    end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.OnMessage := AppMessage;
end;

end.