'masthead-fixed' class causing problems on reloading a scrolled page

While developing a child theme of Twenty Fourteen, I’ve encountered a problem with the masthead-fixed class of the body. This is how the problem is created:

1. I’m resizing the page to trigger the breakpoint that hides the full navigation and makes the menu-toggle button appear.
2. I’m scrolling down the page.
3. I’m reloading the page.
4. I’m resizing the page to something bigger than the breakpoint.
5. Voilà: My primary navigation now has fixed positioning, because the masthead-fixed class is still assigned to the body. The class isn’t even removed on scrolling back to page top.