This happens even on your site - When you press the My Profile menu item - the Profile top moves up under the Header - forcing the user to roll the mouse wheel down to see the Menu bar and/or header.
This happens on my site, and it's very annoying.
Yes, this is designed to work that way. The profile page header is big so it automatically moves your screen to the profile tab content. If you click on Activity menu you're going to see Activities not the top menu so it's better to scroll your view to activities. Without this function you'll always see your website header and scroll it bottom.
I appreciate your answer that the software was 'designed' to act that way. So, What can I do to make it look like this (my header doesn't disappear like yours does) - I would like the entire forum to remain in the block it's been assigned by my theme (Gecko)...