Sticky diff file headers within MR tab broken in Safari

When viewing a diff tab in a MR, the file headers appear to be displaced by about 100px and are not "sticky" as they should be. I'm not sure if the position: sticky polyfill is broken here or if there is some other problem. This appears to be a regression in %11.8

Noticed in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9468/diffs with Safari 12.0.1