As [0, 1] shows the right side of a thread is either fixed or unused, any particular reason for this engaging Flow design because LQT did in fact (as far as I remember) use the full browser width!
[0] https://cloud.githubusercontent.com/assets/1245473/8619452/6cdc918c-2717-11e5-8956-bd224af5103f.png
[1] https://cloud.githubusercontent.com/assets/1245473/8619617/ba5bbd88-2718-11e5-9f83-60ee72b2ef19.png
In the main board, you can use the full window width by closing the "About this discussion" panel.
Web sites typically use narrow columns because they're better for reading long text. Jumping to the next line is easier in them when reaching the end of each line.
I can't follow this argument, most website I visit doesn't show the behaviour as outlined in the attached screen shots. Also asking me to "closing the "About this discussion" panel." isn't really intuitive nor should it be compared to the previous LQT where Flow claimed to be an equal replacement for it. I would have expected that Flow will appropriately resize in a modern browser but this isn't the case as shown in [0, 1].
The text does resize and reflow if you make the window narrower, it just has a maximum width of 2/3rds of the screen by default. This is fairly common in current design trends for blogs and news sites. Several users complained about the text not using the whole screen, so the developers added that possibility with an option to collapse the unused panel to the right.
I think I understand MWJames concern, which is regarding the narrow span of non-responsive-width, between 895px and 1165px - I've filed this as phab:T107241.
