Keyboard shortcuts steal focus from reviewer dropdown in MRs

Summary

When I try to type a name in the Reviewer dropdown on an MR, the page itself is still picking up the typing as keyboard shortcuts rather than an entry in the search users.

Steps to reproduce

• On any GitLab MR, open the reviewer dropdown and immediately start typing. Notice that you're navigated away from the dropdown.
• This only happens on initial mount of the dropdown component. On second opening, the bug is not present.
  • Possible focus issue while mounting/loading dropdown items

Example Project

GitLab

What is the current bug behavior?

keyboard_shortcut_bug

What is the expected correct behavior?

The dropdown input should be receiving the typed text

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

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`)

Possible fixes