Sidebar broke with upgrade to 2.2 and now fixed

I have TwentyTwelve theme installed, with a child theme, and upgrading to 2.2 from 2.1 broke my sidebar on my blog main page. The sidebar appeared below the list of posts rather than beside them (still on the right, but below them). It was in the right position on individual post pages, archive pages, category pages, so it was just the main page that wasn’t working.

Even when I removed all my child theme’s CSS (using Firefox developer tools) the sidebar remained in the incorrect position even with just the TwentyTwelve CSS.

Looking closer I found that my blog main page was affected by a “body.full-width .site-content { width: 100% }” css rule that was not on the other pages. This was causing the problem.

Going to Pages > My Blog Main Page > Page Attributes in the WP admin I did not find a “Template” selector like on other pages. For some reason it did not appear there. Going back to “Pages” and clicking “Quick Edit” for my blog main page I saw a “Template” selector which was set to “Full-width page template, no sidebar”. Changing that to “Default template” fixed the problem.

Thought I’d post it here in case others are affected.