Happens on both Firefox and Chrome on Windows, at 125% scale. After the left and right body margins grow beyond 300px each, the header starts moving into the main text.
Weirdly enough, I can only replicate that behavior when I go to scale=80% and below, but it also varies based on the exact zoom. Perhaps if you zoom in one level below or above 125% it will resolve? I am admittedly not really good at CSS or HTML.
Sorry, I meant 125% screen scaling from the OS, not browser zoom. But I just tested and it also happens with no scaling. As I resize the window the header links slowly move down, until they overlap the text.
