Markers for comments on issues can hide detail and can't be hidden themselves
Consider https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3650/diffs:
Underneath that 1
marker is a small arrow, which is now invisible. If i'd know, I would have put the marker just above the arrow, rather than on it, but there's no way to move it (except to delete the comment and re-do, perhaps?)
Ideally, there'd be a way to hide all markers. I love that I can do it at all though!
/cc @victorwu