It's possible to drag and slide to make some panels bigger and smaller. There's also a button to collapse it down to just the icons to toggle between it
Which is nice because I'm not used to having vertical tabs
I have the tab bar set to expand when I hover over it. But when I split tabs to see two pages at once, I don't seem able to make them take up different proportions of the screen. The split tabs feature was what made me want to give it a spin, so I hope it is possible.