header 1px artifact in Safari

Hi,
Not sure this is the right place to mention this, but I couldn’t find any other forum for it.

I just started converting an old twenty eleven site to twenty fifteen and I’m loving the look of it. I use this site (owls.wpstudent.net) as an example for the class I teach on learning how to use WordPress and I’m glad to finally upgrade. (Frankly, been disappointed with the default themes since eleven.)

Anyhow since I’m going to be using this as an example for future WordPress users I’d like it to be as good as possible.

I noticed that on the right hand side of the header, while in sidebar view, there is a 1px artifact that is persistent. I tried several header images of several dimensions and the same effect happens.

Out of curiosity I checked out on WordPress.com where you have examples of folks using the twenty fifteen and sure enough, same thing.

I checked this out in Chrome, Firefox and Safari and it seems to be specific to Safari.
my site is owls.wpstudent.net and the other sites you can find in the WordPress.com page for the theme.
Any adjustments to the theme to adjust this would be appreciated.