Bartik theme has a responsible primary navigation built in with three display sizes for mobile, pad and desktop.
In mobile display either a "Show" or a "Hide" menu-toggle bar are being provided to show or hide the primary menu. These two bars are not needed in pad and desktop navigation display as the primary menu is always visible in those display sizes.
Bug
By resizing the browser window you can show the primary menu in mobile mode, then resize the browser for pad or desktop display size and have the menu-toggle hide bar still being displayed, because it is not properly hidden by CSS. On mobile and pads this can be a problem in those cases when portrait and landscape display would cross over mobile and pad/desktop navigation sizes.
Reproduce on desktop / laptop computer
1. Install vanilla Drupal 8 (8.0.x - Feb 2nd, 2016) with standard profile and Bartik theme enabled.
2. After installation resize the browser to display mobile navigation.
3. Click on "Show - Main navigation" or the hamburger icon.
4. Resize the browser window for pad or desktop navigation display.
5. Evidence: "Hide - Main navigation" bar is still visible.
Solution
Apply patch of first comment in this issue.