Stop compression of header when scrolling down

On the desktop version of my site, the header compresses as I scroll down–and it’s so choppy. I would prefer if it acted like a normal element as my users scrolled. On the mobile version, for instance, the header gradually disappears as I scroll without compressing. That’s what I want on the desktop version.

For what it’s worth, I’m new to WordPress and I don’t have knowledge of coding yet. But I’m also a pretty smart cookie, so I should be able to follow your instructions.

Thank you, guys. I really appreciate the altruism you guys show here. ??