Finally. I have a big monitor and the line width is insane, glad they made it a more reasonable column layout, moving my head from end to end was getting very annoying that I had to use custom CSS instead.
Why can't you just resize the window to a width that is comfortable for you?
I use all my applications in full screen mode usually, so resizing just for a single tab or moving Wikipedia into its own window would be annoying too.
They could have made a responsive multi-column layout to keep the readability of reasonable line lengths while keeping information density high.