In reply to Nick Halsey.

With regard to the following statement:

"probably worth noting that we’re planning on bumping out the Customizer controls area to almost full-width when we add theme-install"

Do I understand this correctly that you will make the customizer side panel almost fullwidth all the time? If so how can you then see the live preview of the customizations you are making? Or, are you meaning that the customizer will grow in width if you are browsing and selecting a theme specifically?