I would say that the Forum link should open in a new window. As a user is quite likely to get bored watching their longer prints, and may follow the forum link out of curiosity. Losing your progress window is a bummer in that case.
Not seeing that option in the webapp. If you mean change it for my whole browser, I can make it open all links in new tabs all the time, but wouldn’t want to.
You are probably right. My age-old habit is just to right click > open in new tab when I hit something of interest, not knowing if the target is set to _blank or not.