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Сообщение от Decoding

если еще актуально (да и, мало ли, может кому пригодится)...

Ой как неправильно.

Код:

unit Unit1;

interface

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

type
  TPanel = class(ExtCtrls.TPanel)
  protected
    procedure Paint; override;
  public
    property Canvas;
  end;

  TForm1 = class(TForm)
    Panel1: TPanel;
    Button1: TButton; 
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TPanel }

procedure TPanel.Paint;
begin
  inherited Paint;
  Canvas.Pen.Style:=psDash;
  Canvas.Rectangle(10, 10, ClientWidth-10, ClientHeight-10);
end;

{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject);
begin
  Panel1.Canvas.MoveTo(10, 10);
  Panel1.Canvas.LineTo(ClientWidth-10, ClientHeight-10);
end;

end.