procedure TForm1.FormResize(Sender: TObject);
var P : array [0..9] of TPoint;
    dm:dword;
begin
form1.BorderStyle:=bsSizeToolWin;
form1.BorderWidth:=0;
dm:=form1.Height-form1.ClientHeight;
P[0] := Point(0,0);
P[1] := Point(form1.Width,0);
P[2] := Point(form1.Width,form1.Height);
P[3] := Point(0,form1.Height);
R := CreatePolygonRgn(P, 4, Alternate);
P[0] := Point(2,dm);
P[1] := Point(form1.Width-2,dm);
P[2] := Point(form1.Width-2,form1.Height-2);
P[3] := Point(2,form1.Height-2);
R1 := CreatePolygonRgn(P, 4, Alternate);
CombineRgn(R, R, R1, RGN_DIFF);
SetWindowRgn(form1.Handle,R,true);
end;