Okay--I just discovered what might be a header issue regarding responiveness:
When a site title and tagline is entered, the header appears in its full height. But when the site title and tagline is left blank, the header only opens at half its height, and then the height gets cropped badly when it's opened wider on a desktop browser.
When the site title and tagline is entered, the header image remains at its full height as the browser narrows (simulating use on a tablet or phone), with the width of the header image being cropped. It's the opposite when the site title and tagline are left blank.