Mention picker won't display if comment box is blow viewport

Summary

When @mention a user in a comment the picker won't appear if the comment box extends below the viewport.

Steps to reproduce

Begin writing a comment on an issue
Scroll to the point where a portion of the comment box extends below the view port
Try @mention a user in that comment

Example Project

/gitlab-org/gitlab/

What is the current bug behavior?

No user picker when @

What is the expected correct behavior?

A user picker popup

Relevant logs and/or screenshots

2020-09-03_15-18-52

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

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)