Vlad In that arrangement, what happens when you select Bookmarks in the leftmost bar? Does it replace the right bar? Cause I could see that not clicking as it's an expanded layout for something users wouldn't interact with much (except the researchers that like tab groups so much). Bookmarks + open tabs as sidebars might be more interesting though. Its annoying switching, especially with width shifting.
I guess with the upcoming compact top bar (if its like the mockups you posted) might be enough for me to have just bookmarks as sidebar.