I think the 'error' may have been related to how much of the header image is visible before and after scrolling down a page.
Without Troyane's code
For me, more of the header image is visible before you scroll down. When I scroll back up, some of the header image is cropped at the top.
With Troyane's code
The same amount of header image is visible before and after scrolling, however its now cropping the top of the header image permanently for me in both instances.
My header image is loaded at the recommended size 1600 X 400.
While I'd prefer to the header image to not be cropped, its not a big issue for me that it crops it when I scroll back up. I had noticed the issue and wondered if this fixed it.