It would be ok if the header was kept just above the viewport edge and kept there (ie. slowly getting into view) when scrolling up. You still have near-instant

It would be ok if the header was kept just above the viewport edge and kept there (ie. slowly getting into view) when scrolling up. You still have near-instant access to the header, but it doesn't jump out at you at the tiniest hint of scroll up. The problem with "scrolling back down to re-hide" is that most of the time, scrolling down enough to hide the header also scrolls the start of the content out of view. That means you need to scroll up enough to account for both the header height and the underlying, obscured content to stay within the viewport once the header disappears. I never once in my life got it on my first try, and it often takes 3 or even 4 tries to put the first line of the paragraph at the beginning of the viewport without it getting obscured. To me, the idea itself is not that bad and there are some usability gains to be had, but the implementations are just horrible, lazy, and user hostile.