The idea is to take advantage of the whole viewport, keeping a column not too wide (~70 characters) for the content to have good readability. I had thought of an option to collapse the first sidebar and show it only with icons (which I would like to use elsewhere, too). Thanks for resurrecting this idea!

Then, like you can see in the Figma, there is also a "zen mode", where you see only the main content and the menu is dimmed.