Signup: 1st screen is mostly blank when trying to add a 2nd blog to an account with one blog

[designsimply]

Steps to reproduce:

1. Log in to WordPress.com with an account that currently has only one blog
2. Click "My Site" in the top toolbar
3. Click "+ Add New WordPress"

Result: a mostly blank screen is shown and I cannot continue with setup.

Notes: the problem does not happen for accounts with two or more blogs nor does it happen if you load /start/themes directly while logged in to an account with 2 or more blogs—you must hit "My Site" followed directly by "+ Add New WordPress", which is a common flow.

^{Safari 9.0.1 on Mac OS X 10.10.5 logged in as user552}

Looks like it might be a problem with height:

^{Chrome 46.0.2490.80 on Mac OS X 10.10.5 logged in as user552}

This span looks sketch:

^{Chrome 46.0.2490.80 on Mac OS X 10.10.5 logged in as user552}

Is it possible we're not including some CSS where we're supposed to?

/hat tip m for reporting the issue

[nb]

Not the core, but closer to the core of the problem: the difference between a working and not working version is focus-sidebar vs. focus-content class on the .layout element.

[aduth]

.focus-sidebar applies an overflow: hidden; which, when removed, allows the content to render correctly. May need to use layoutFocus to update the focus to content when choosing to add a new site?