I am quite fascinated by the navigation solution of the site  https://www.simple.com .

As soon as you go through the main-menu item "Meet Simple", the submenu always stays on top of the site beneath the main menu and kind of docks to the main menu as soon as it would normally disappear by being scrolled out of the viewport.

And, as soon as the part of the website which the subnavigation corresponds to, is scrolled out of the view, the subnavigation gets scrolled out as well.

I really, really would like to get this on my own site, but I don't really have a clue how they solved this docking/undocking effect as the user scrolls down the page. Can someone give me a hint? Would be appreciated a lot!! :)

Regards,

beyondHor1zon