Charts not rendered when Startup.CSS enabled due to cloaking

Summary

Refers to this discussion in #235497 (closed) and it's caused by !39115 (merged)

Consistently reproducible with Chrome and Edge, less with Firefox and Safari (Safari is still a mystery to me).

Steps to reproduce

Steps to reproduce:

1. Visit this page with cached assets and startup_css cookie to false (or not set)
2. set the startup_css cookie to true
3. refresh (with Disable Cache checked) or Hard-refresh
4. the charts are not showing

Note that a normal refresh (or refreshing with Disable Cache unchecked will show the page correctly).

Example Project

Can be tested on staging or locally (prod is now startup.css disabled).

What is the current bug behavior?

As shown in the videos, the charts are not rendered.

Chrome	Edge
charts_startup_chrome_480	charts_startup_edge_480

Firefox	Safari
charts_startup_firefox_480	charts_startup_safari_480

What is the expected correct behavior?

The charts should be rendered

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)

Possible fixes