It seems that it disappears (rolls off the top of the screen) when you scroll down, and reappears when you scroll up.
There's little or no "inertia" in that behavior though, so depending on your mousewheel/track-pad/touch scroll behavior that can seem kinda flaky. A tiny scroll-up action brings the sticky header back. I have noticed other sites that do that can be really annoying on mobile, but on a MacOS laptop at least this site seems to do it pretty well (IMO).
It might benefit from not re-displaying the header until you've scrolled up a little more than it does now (again, IMO). I.e,. maybe have a threshold of a certain number of pixels (>1) or a certain velocity of scrolling-up before the header re-appears.
UPDATE: Playing with it a little more I do think it is a little flakier than that. There's something weird about the marquee image that makes the header re-appear for reasons I don't understand. If you use the arrow keys for scrolling (so there's no chance of an accidental "bounce" when you stop moving your fingers) the behavior is a little more obvious and consistent, but you can see that when scrolling from the top of the page the header disappears then comes back briefly for reasons that aren't clear to me.
Clever or subtle UI/UX behaviors on the web are hard.
There's little or no "inertia" in that behavior though, so depending on your mousewheel/track-pad/touch scroll behavior that can seem kinda flaky. A tiny scroll-up action brings the sticky header back. I have noticed other sites that do that can be really annoying on mobile, but on a MacOS laptop at least this site seems to do it pretty well (IMO).
It might benefit from not re-displaying the header until you've scrolled up a little more than it does now (again, IMO). I.e,. maybe have a threshold of a certain number of pixels (>1) or a certain velocity of scrolling-up before the header re-appears.