White line on the top of my retina display

Hi,

I have noticed a white line (photo 1) on the top of my retina display (MacBook Pro 13" retina mid 2014) when I open the Launchpad. In other conditions the line seems to be absent. For example, if I run App Exposé (photo 2), I don't notice any line on the top of the screen. Is it a normal behavior for the retina display? Is it a hardware or software issue?

Thanks in advance,

regards,

Luigi