Thanks for the live site link. I don’t seem to be experiencing the same issue as you. Have you tried testing your live site on a couple browsers?
Also, I notice your site’s fonts “flash” every time you load a page. Perhaps try this trick that keeps the site blank until all fonts are loaded (thought the site will flash white instead - your choice on that tradeoff.
Another approach I recommend you check out is to use a single Tab component on a single Page rather than splitting all your pages up across 6 manually created tabs. Doing this will effectively solve the shifting topbar issue you mention in this post as well!