MacBook retina 12 inch 2015: text garbled on URL bar of safari What can cause this?
The URL bar is behaving weirdly and cannot be edited
I have had the same issue  happen to me, once (I also have the same laptop)....I just rebooted and it never happened again. 

I'm unsure if it's even a machine/OS X issue or a scripting error.

Hopefully that solves your same problem.