Configuration overlaps between Styles and other CKE5 plugins

So this is to fix the edge case reported by @DanielVeza at #3222797-97: Upgrade path from CKEditor 4's StylesCombo to CKEditor 5's Style. Credited them!

1. +++ b/core/modules/ckeditor5/src/Plugin/Validation/Constraint/FundamentalCompatibilityConstraintValidator.php
   @@ -207,7 +207,6 @@ private function checkAllHtmlTagsAreCreatable(EditorInterface $text_editor, Fund
   -        assert(count($matching_plugins) === 1);
   

   🤔 Removing this assertion is a significant change: it means that multiple plugins may be adding support for a particular tag.

   We should not need this part.

2. +++ b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
   @@ -1253,6 +1253,125 @@ public function providerPair(): array {
   +    $data['INVALID: Style plugin configured to add class already added by an other plugin'] = [
   

   👍

3. +++ b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
   @@ -1253,6 +1253,125 @@ public function providerPair(): array {
   +    $data['VALID: Style plugin configured to add new class to and already restricted tag'] = [
   

   👍

4. +++ b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
   @@ -1253,6 +1253,125 @@ public function providerPair(): array {
   +    $data['VALID: Style plugin configured to add class to an element that already allows all classes'] = [
   

   👍 … but (!) this is only testing the case of a concrete plugin supporting a particular element, it's not testing the case of a SourceEditing-provided element.

   I think we need more test coverage.

If it weren't for that one line and the missing extra test coverage, I'd RTBC 😊

This addresses #21.1 (a single line revert) and #21.4 (more test cases).

Note: the single line revert is trivial, and so are the test cases. Explaining the test cases was more difficult than writing them. So if @nod_ +1s the added test cases, I'm comfortable RTBC'ing.

I was pretty worried about #3222797: Upgrade path from CKEditor 4's StylesCombo to CKEditor 5's Style going in with this edge case happening, but it was also a bad idea to expand the scope of that issue further. It happens to make this solution one of the easier CKEditor 5 patches I've reviewed. Fix makes sense, as do the tests.

The fact that this problem was happening indirectly showcases how robust validation is in the CKeditor 5 modules. For every hassle such as this one, we've been spared a dozen larger ones.

Committed to 10.1.x and cherry picked to 10.0.x and 9.5.x. Good fix folks!

Amazing team! ❤️

I was pretty worried about #3222797: Upgrade path from CKEditor 4's StylesCombo to CKEditor 5's Style going in with this edge case happening

That issue went into 9.4.x. Should this one as well?

The fix should indeed still be backported to 9.4.x, @Eric_A!

Will provide a patch. Currently not cleanly cherry-pickable. So determining what the optimal cherry-pick order is …

#22 still applies cleanly to 9.4.x too! Test queued 👍

Committed b7b5e1a and pushed to 9.4.x. Thanks!

Restoring previous status.

I am running php 8.1.14 and Drupal 9.5.3. I am still unable to add a span with class in the styles plugin.

Here is the error message after trying to add span.test_class|Test Class to the Styles configure textarea

Error message
The Style plugin needs another plugin to create , for it to be able to create the following attributes: . Enable a plugin that supports creating this tag. If none exists, you can configure the Source Editing plugin to support it.

@jksloan2974 I recommend bringing up your concern in #38 in Drupal Slack or opening a new issue describing the steps to reproduce. Issues are rarely re-activated, and in those rare instances it's within a few days of it being completed. Since it's been 5 months, this will not re-open to address your concerns, but if it were a new issue (or question in Slack), it can get some attention.