I found the background for each line of code wasn’t expanding to fit the scroll-area properly when a wide piece of code was being viewed. This has been declared as an official bug and the workaround suggested by brucknerite works nicely.
I have made a few tweaks to his fix so that it displays properly on WordPress. I’ve attached the culprit files to the end of this post.
* Having troubling getting shCore.js to compress using DEP. It’s probably just a simple syntax niggle, so I will investigate this when I’m less busy!