Adding new text format gives Uncaught TypeError: f.format_tags.split is not a function

Nice catch! Manually tested and I was able to replicate the bug and confirm that the patch works great. We should have some testing for it.

Just a nit:

+++ b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
@@ -342,14 +342,15 @@ public function getButtons() {
+   * @return string|false

s/string|false/mixed

Still encountered this issue at 8.6.

Converted patch to 8.6/8.7
Added comment from #6

Reproduced. Ran into it over at #3060749: entity_embed_filter_format_edit_form_validate() Logic is Faulty.

Best of all: there is an explicit JS test over there! We can reuse that here, to justify the core fix :)

So I think #13 means this already had to go back to needs work. But I have some additional nitpicks. The only one actually being actionable being the second.

1. +++ b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
   @@ -116,8 +116,8 @@ public function getConfig(Editor $editor) {
   +    if (in_array('Format', $toolbar_buttons) && $format_string = $this->generateFormatTagsSetting($editor)) {
   +      $config['format_tags'] = $format_string;
        }
   

   I personally don't like doing assingments in if statements, however I don't see a way to do this in a cleaner way, without introducing more nesting. So let's keep it like this in this case.

2. +++ b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
   @@ -344,14 +344,15 @@ public function getButtons() {
   +   * @return string|false/mixed
   

   false/mixed is not a valid type I think, we should probably keep just the false and remove the /mixed.

3. +++ b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
   @@ -344,14 +344,15 @@ public function getButtons() {
   -      return [];
   +      return FALSE;
   

   So the docs say that this used to return an empty array or a string. That doesn't make sense.

   I was going to complain about introducing a mixed return type where it might've been avoidable, but it looks like this was already broken.

Made patch working for > 8.8.7

It was still on needs work because tests are needed.

The solution fixes the problem. I understand why we hold up merging this waiting on tests ... but this is a pretty fundamental broken feature (adding buttons to the editor) that takes some skill to work around. Can we consider merging this without tests?

Adding a failing test case to demonstrate the reported issue. CKEditor plugin settings are not visible without applying this patch

Also this should address #14

Manually reproduced, with this patch applied it works correctly! 👍

Thanks, @raman.b for adding the failing test case!

Committed and pushed 190aae1079 to 9.3.x and c051ff7508 to 9.2.x. Thanks!

Backported to 9.2.x since this is a low-risk bug fix.