Collapsible Reply Context Menu Pops Under Issuable Sidebar

Summary

Steps to reproduce

1. Navigate to a merge request
2. Open the Changes tab
3. Locate a comment with a collapsible reply
4. Open the contextual menu (three vertical does)

Example Project

!55858 (diffs)

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

wabXif-tofdic-2pogvi

Possible fixes