I would like to propose a new feature for vertical tabs where it is possible for the sidebar with vertical tabs to auto-hide and re-appear once you hover your mouse cursor over the left-edge of browser.
This feature is available in Arc Browser and I really like it, as it gives you a really clean design and whole screen to browse websites. In 'Standard' and 'Compact' tabs modes, the UI is already cluttered and not to mention working with many tabs is nearly impossible to see what you really have opened.
EDIT: I just noticed that this very same Forums have the feature I am describing! Once I posted this suggestion, I noticed that the sidebar is appearing and hiding automatically, depending on my mouse cursor position. So basically, this same feature, but for Vertical Tabs window!
A user would have an option in Settings to toggle the 'Auto-Hide' sidebar for Vertical Tabs. When toggled ON, the sidebar with vertical tabs automatically dissappears once the user moves the mouse cursor out of left-edge of Orion Browser.
To bring the sidebar back into view, user would move the mouse cursor to the left-edge of Orion browser and the Vertical Tabs sidebar would hover-over the web-content on the left-side for the user to interact with. Arc Browser does this.