Game Mechanics: FD + Open Trade Window + CoS = Trade Window stays open?

[Anichek]

Talked to someone tonight who was asking if they could CoS after opening a trade window and still be able to be invis while vendoring to do a quest turn in. I thought no - any invis would break the trade window from being open. Advised them to try a FD turn in....

...they were able to FD, then open the trade window, then stand and CoS to invis, and the trade window stayed open.

Not sure if this is a true bug, but posting for review in case it is.