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public
    { Public-Deklarationen }
    procedure WMSysCommand(var Msg: TWMSysCommand); message WM_SYSCOMMAND;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WMSysCommand;
begin
  if (Msg.CmdType = SC_MAXIMIZE) then ShowMessage('Maximiert');
  DefaultHandler(Msg);
end;                    

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Case Msg.CmdType of
  SC_MAXIMIZE: ...
  WM_NCLBUTTONDBLCLK: ...
end;