That is a legacy of our desire to have a mobile builder that worked well on a mobile application. Switching tabs in a mobile device seemed harder. So we kept the whole experience in one tab. We recognize now that few or none are actually building pages on a mobile device. So we need to change this. I'll add it to the list.
When I'm building pages, I usually open the builder in another tab and preview it, and then swap back and fort between tabs, refreshing as needed.
The Mobile builder was deprecated in the Brooklyn release, so we won't be doing further development on it.
The recommended method for building mobile pages is to use the App Composer and responsive components; in Brooklyn we added a Deck component and we're improving the responsiveness of individual components on an ongoing basis.