After I took the screenshots I realized it is likely you are not using FF - sorry, but hopefully the general idea should lead you to solution anyway.
F12 to bring out the dev tools
You are looking for a <div> with the “right-col” class (as in right column).
The easiest way to find it (for me) is to just activate the element selection tool (1) hover over the right column until the whole column is highlighted and then left-clicking.
The number you need to adjust is the flex-basis (default 25% as in quarter of the browser area), and you probably want to change it to 50%
Either way this actually sounds like a cool option to have in the interface and not terribly hard addition doable from the frontend so if there are any coders looking for a starter challenge our github is here: https://github.com/online-go/online-go.com/
Also also, note that you can expand the tree or comment boxes by the lower right edge. I did not notice until like a week ago 