Random horizontal scrollbar in code macro when resizing sidebar in Chrome

I'm using Windows 7 and Google Chrome (34.0.1847.116 m). I tried the web page in Internet Explorer 11 and Firefox and couldn't reproduce the issue there, I then tried in a Chrome Incognito tab but the issue was still reproducible - which is very strange.

As I expand/contract the side bar the scroll bar appears/disappears randomly - it seems very inconsistent, I've attached an animated gif of the behaviour.

Thanks,
Oliver