Facebook "Like" pop-up is cropped

Love the plugin, but when you click on the “Like” button for Facebook, the resulting pop-up which appears below is cropped to the width of the Facebook section.

A very similar problem seems to have been an issue before

http://wordpress.org/support/topic/facebook-likecomment?replies=5

But that says, “We tried hard and finally, found a permanent solution, from our end, so this problem, will never happen again.” My guess is perhaps that this is a clash with my theme’s style.css, but I don’t know what I would add to fix it.