Ahah, yes, that's how backward compatibility works sorry, my own scripts would break on -1 if not modified but since I'd be modifying them, I forgot.
Then a solution would be to improve $sfile to allow it to return a different value when the dialog is canceled.
Something like $sfile().cancel, which would return -1 if the old $sfile were to return $null (which as far as I know, is only possible when the user cancel the dialog), and would still return $null for reserved folder
