Centered next button moves slightly when clicked (Back button hidden)

Hello,
I'm working on a project where the back button is hidden and the next button is centered in the header of each survey page, and the header is locked so it stays at the top of the page when scrolled. Everything is working as I would like except when the next button is clicked it jumps slightly to the right side as if to accommodate/split center with the hidden back button. This does not happen on pages where there is a back button shown. The centering of the buttons is done in the Header CSS and the previous button is hidden using HTML to prevent flashing as per previous posts.
Is there some way to prevent this movement when clicked? This is something I could live with, but I'd rather find a solution.
I'd be happy to offer more context and welcome any suggestions.
Thanks!
CatherineO