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Код:

unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm2 = class(TForm)
  private
    { Private declarations }
    procedure WMSYSCOMMAND(var Msg: TMessage); message WM_SYSCOMMAND; 
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

{ TForm2 }

procedure TForm2.WMSYSCOMMAND(var Msg: TMessage);
begin
  if Msg.WParam=SC_MINIMIZE then Application.Minimize else inherited;
end;

end.