procedure TForm1.Button1Click(Sender: TObject);
  var
    x,y: Integer;
      begin
        x:=0;
        y:=0;
          while x < Form1.ClientWidth
          do begin
            Form1.Canvas.MoveTo(x,0);
            Form1.Canvas.LineTo(x,Form1.ClientWidth);
            Application.ProcessMessages;
            x := x+10;
          end;
          while y < Form1.ClientHeight
          do begin
            Form1.Canvas.MoveTo(0,y);
            Form1.Canvas.LineTo(Form1.ClientHeight,y);
            Application.ProcessMessages;
            y := y+10;
          end;
      end;