Unless the user is actively navigating, the header is dead weight. The header should hide on scrollDown and reveal on scrollUp. Let the content breathe.
This one I actually hate. Often I just want to scroll up a few pixels, either to satisfy a mild compulsion or to align the content so I can see most of it. This is completely ruined if the navbar pops back in. Leave it at the top of the page, where it belongs, not at the top of the viewport!






Even if it did, how would any user ever find out about this obscure feature?