Fix left sidebar overlap when scroll on long menu
Fixes #17566 (closed). Caused by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4139. Smaller fix compared to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4164
Signed-off-by: Dmitriy Zaporozhets dmitriy.zaporozhets@gmail.com