Fix left sidebar overlap when scroll on long menu

Fixes #17566. 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

cc @iamphill @jschatz1 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !4165