Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

type
   TForm1 = class(TForm)
   private
     { Private-Deklarationen }
     FOldHeight: Integer;
     procedure WMNCRButtonDown(var Msg: TWMNCRButtonDown); message WM_NCRBUTTONDOWN;
   public
     { Public-Deklarationen }
   end;

 var
   Form1: TForm1;

 implementation

 {$R *.DFM}

 procedure TForm1.WMNCRButtonDown(var Msg: TWMNCRButtonDown);
 begin
   if (Msg.HitTest = HTCAPTION) then
     if (ClientHeight = 0) then
     begin
       ClientHeight := FOldHeight;
       Application.ProcessMessages;
     end
     else
     begin
       FOldHeight   := ClientHeight;
       ClientHeight := 0;
       Application.ProcessMessages;
     end
 end;