Running Stadia on a Pixelbook in tablet mode still results in a broken portrait view. I've sent feedback on this but it's still an ongoing issue that seems could take a simple fix to force the page when loaded to be landscape only. Unless I don't understand how web development works...
Ultimately it's a pretty minor thing but as Chrome tablets start to become more prevalent (instead of Android tabs) this will get noticed more.
great finding! Please use the feedback feature of your Stadia mobile app to report your enhancement idea directly to the Stadia team. 🙂