GitLab Migration UI hangs when updating namespace information

Summary

When using GitLab Migration UI groups selector, there seems to be a some js running that prevents the page from interacting fluently. Page freezes for a bit of time making it difficult to use.

Demo: note the cursor stops blinking when I type 2 and delay in the namespaces selector opening. the page just freezes

migrationui

Steps to reproduce

Go to groups/new page and select Import group
Enter source GitLab URL and token to view source instance groups page
Try updating namespace information / use namespace selector to see the page sometimes freezing

Example Project

What is the current bug behavior?

What is the expected correct behavior?

The UI page shouldn't freeze when updaing namespace information/using namespace selector

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

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)