When trying to reference an existing entity that has a very long title (title can be up to 255 characters), a validation error is thrown because the text added by the autocomplete js code will be longer than the maximum field size, which is set to 255. This is caused by the extra characters needed for the id to be included in the string (in the format "<title> (<id>)").
Use the same maxlength used by the \Drupal\Core\Field\Plugin\Field\FieldWidget\EntityReferenceAutocompleteWidget, which is 1024.
Test maybe.
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None.
Needs work
3.0
Code
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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