Thank you for sharing this concept, tr3k.
I agree it could be a nice option to add, but out of curiosity, did you try moving the main tab bar below the URL field as well?
This way you would avoid this split happening with the panel bar, which now get's cut off by the secondary tab bar, as well as the URL visually splitting the relationship between the main tab and sub tabs.
Obviously the UI still jumps as the entire page content jumps down due to the new row being added, so we can't avoid that.
I can't promise we'll add this as a setting, but I could rather see this as part of a fully customizable UI, where any component could be moved freely. So in stead of individual settings, which we already have too many of IMO (not great for discoverability), you'd have the option to put it wherever you want; maybe even at the bottom, above the status bar?
Keep exploring! And share your custom CSS with the community 🙂