I'm using a sidebar with Twenty Thirteen and the Disqus comment box spans the entire screen, cutting across the sidebar. Example:
I see that this issue was mentioned back in April:
but the example linked to no longer helps, as the site evidently has removed Disqus.
This blogger mentioned the problem in the forums here and has evidently fixed it:
but hasn't reported back. I was asked to create a new thread on the issue.