[random test failure] CKEditor5MarkupTest::testStylesAndScripts

Problem/Motivation

See https://git.drupalcode.org/issue/drupal-2840283/-/jobs/11289519

     CKEditor5Markup (Drupal\Tests\ckeditor5\FunctionalJavascript\CKEditor5Markup)
     ✔ Attribute encoding
     ✔ Filter html allowed global attributes
     ✔ Comments
     ✘ Styles and scripts
       ┐
       ├ Behat\Mink\Exception\ExpectationException: The string "const example = 'Consider this string:
    

Steps to reproduce

Run repeatedly (assuming ddev and selenium setup locally):

ddev exec vendor/bin/phpunit -c core \
  core/modules/ckeditor5/tests/src/FunctionalJavascript/CKEditor5MarkupTest.php \
  --filter testStylesAndScripts

Local results
Two consecutive runs produced different results:

• First run failed at CKEditor5MarkupTest.php:346, reporting that the first <script> test case was absent. It stopped after 9 assertions.
• The immediate second run passed with 19 assertions. GitLab failed on a different script test case, suggesting the markup itself is not the determining factor.

Likely race condition
Each iteration saves the node and immediately inspects the response:

$page->pressButton('Save');
$assert_session->responseContains($expected_content);

There is no wait for post-save navigation/rendering to complete. The assertion can therefore inspect the node form or an incomplete response.

Proposed resolution

Wait for an element or text unique to the saved node page before asserting its HTML, for example:

$page->pressButton('Save');
$this->assertNotEmpty(
  $assert_session->waitForText("Style and script test - $test_case_name")
);
$assert_session->responseContains($expected_content);

Similar waits already exist elsewhere in CKEditor5MarkupTest for save-related random failures associated with #3520036: [random test failure] CKEditor5MarkupTest.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet