* Having the @username anywhere in the tweet preserves the pointer.
* Deleting the username clears out the pointer.
* Deleting the username, and retyping it (at the end of the tweet) restores the pointer.
The last scenario makes me think that the validation is server side. It seems that Twitter ensures that the tweet still mentions the author of the replied-to tweet or it will clear out the pointer.
Replies are triggered by two factors. An `in_reply_to_status_id` field that is filled in behind the scenes and an @mention of the original author in the text.
* Having the @username anywhere in the tweet preserves the pointer.
* Deleting the username clears out the pointer.
* Deleting the username, and retyping it (at the end of the tweet) restores the pointer.
The last scenario makes me think that the validation is server side. It seems that Twitter ensures that the tweet still mentions the author of the replied-to tweet or it will clear out the pointer.