You cannot open a popup except in response to a direct user action.
Since you delay the window.open until the post reply finishes, it is no
longer directly in response to the user's click, and therefore the popup
blocker will stop it.
This will happen for everyone, and you cannot change the behavior. You
could try opening the window before you submit the post, and only filling
it in when the post returns - just move the
up one to just before