Thanks you for your answers, seems to works good with +b switch ^^.
Code:
alias winfbug {
  window -pfh +b @test -1 -1 $1 $$2
  drawrect -f @test 2 1 0 0 $1-
  drawsave @test test.bmp
  window -c @test
  echo 2 -s /window -f !bug: $1-2 -> $pic(test.bmp).width $pic(test.bmp).height
}
But idealy, /window -f should be working in all situations. (An application should adapt itself to OS, not OS -> Application).