...reduce a form by right-clicking on the titlebar?

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;