[ecfisa]

Hola.

Código Delphi [-]
...
var
  R: TRect;
begin
  with ComboBoxEx1 do
  begin
    R := ClientRect;
    SetWindowRgn(Handle, CreateRectRgn(R.Left+2,R.Top+2,R.Right-2,R.Bottom-2), True);
    Invalidate;
  end;
end;

Saludos