The following calls for theme_header_callbacks:
<div class="wrapper">';
call_template_callbacks('th', $context['theme_header_callbacks']);
echo '
</div>
It is nicely wrapped in wrapper class but I think should be given one extra class, useful to cover the overall margin in responsive design.
We got login bar and search bar as default in it but notably, search bar doesn't have its own division. I will suggest that this bar also given it is own division and class.
I will try to create working part of this idea as my Masa addon is inserted in here as well. I think a properly divided theme_header content will help me or any other mod authors to modify this part as well.