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Since upgrading from 5.0 dev to 5.0 release, the main navigation menu does not adjust its position when the admin menu is displayed. It may be that I've incorrectly merged the release changes into my build but I don't think so.
To fix the issue, a 'position: relative;' needs to be added to #main. Then the comment below about positioning the navigation relative to #main's padding in responsive-sidebars.sccs is correct:
/* * Navigation bar */ ... top: 0; /* Move the navbar up inside #main's padding. */