As you all already know, YouTube header is configured to stay visible, affixed to the top, as you scroll down the page to provide a quick navigation to the other sections within the site. If this turns out not to be helpful at all for you, or even annoy you so much because you feel that the persistent header takes up valuable space, you can use Static YouTube Header to fix it.
Static YouTube Header is a userscript. It will make the header stop scrolling with the content once it is installed. Use Greasemonkey to install it in Firefox and Tampermonkey to have it in Google Chrome. If YouTube is open when you install it, refresh the page after the installation is finished.