Global nav overlapped by footer on Firefox and Safari

• Follows https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/362
• Bug on Firefox reported here: https://gitlab.com/gitlab-com/gitlab-docs/issues/298#note_120576299

End of global nav not behaving evenly on Chrome, Safari, and Firefox:

Chrome - expected behavior	Firefox - buggy behavior - nav items behind the footer	Safari - nav being overlayed by footer on scroll ✅ (fixed by !377 (merged))
nav-chrome-right	bug-nav-ff	nav-bug-safari

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Some more info ported from https://gitlab.com/gitlab-com/gitlab-docs/issues/300

Testing on the half screen of a 4K monitor (around 1800x1900), the global nav is cut off and below the footer.

Test page https://docs.gitlab.com/ce/user/admin_area/labels.html

To reproduce, the height must be greater than the page content + the footer combined, as seen in the following screenshot.