Maybe it has something to do with the content in the (right) side-bar (The Bio with picture) or the content of the blog posts (video's with a particular size and setting)?
Those were the among the first things I check. I'm positive that they aren't the problem.
The only issue I see is that being an SEO marketeer, your use of <img> tags doesn't follow standard SEO recommendation. But that has nothing to do with this problem.
Can you reproduce the problem on OP's live site and if so, was there anything different you had to do to replicate it? I can't replicate the issue at all.
3** conditions are needed to reproduce the problem on his live site:
- only happen in Google Chrome, (though admittedly I was lazy and only tested on IE, Firefox, and Chrome :P)
- only happen from 783px to 1284px screen width,
- .primary-navgiation has to be float:right AND .primary-navgiation a has to be white-space:nowrap. (must be both, just changing the float rule to float:none + text-align:right OR changing white-space:nowrap to white-space:normal would fix it)
**(And a fourth condition: must be his site)
The problem is that something is causing an overflow when all 3 of those contions are met. Hence the code I provided at the post above . . . it patches the problem by removing one of the 3 conditions required for it to occur.
I tried to compare his CSS with the original, even took a look at his child-theme CSS here. I didn't find anything suspicious :/