Copying file from parent to child them breaks wordpress function?

I’m baffled by this.

I have a functioning WordPress theme. I created a child theme folder, and have only two files, style.css and functions.css. Both have proper file restrictions and permissions.

style.css:

/*
Theme Name:   Child
Description:  Child Theme
Template:     parent
*/
/* =Theme customization starts here
-------------------------------------------------------------- */

functions.php:

<?php

add_action( 'wp_enqueue_scripts', 'enqueue_parent_theme_style' );
function enqueue_parent_theme_style() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
}

function my_login_logo() { ?>
    <style type="text/css">
        body.login div#login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/login_logo.jpg);
            padding-bottom: 30px;
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

That’s it. Nothing fancy.

Now, when I activate the child theme, everything works as expected. But if I COPY header.php from the parent theme folder to the child theme folder … changing nothing … then the responsive menu functionality disappears on mobile platforms but everything else remains functional. How can this be, if WordPress is supposedly reading in the same exact header.php file in the child theme as is present in the parent theme? Again, I have not changed header.php, only copied it from parent to child theme folders.

Background – wanting to modify header.php to include calls to FaceBook functions, but those aren’t incorporated yet.