If a new field type is added to an entity that does not have any valid widgets or formatters, and this field type is configured as hidden/disabled in the form or view display, this causes \Drupal\field_ui\Form\EntityDisplayFormBase::buildFieldRow() to re-save the config of the form or view display whenever the page is loaded, even though there is nothing to save. We found this when the Bynder module → added a new field type to its media entity type without adding any widgets or formatters. We never set this field to be displayed, and when we had the Configuration Read-Only module → enabled, we got a fatal error when viewing the "Manage form display" and "Manage display" tabs of the media type.
Since in this case, the config save would be a no-op anyway, skip saving the config if the field is not configured for display at all.
Needs work
10.1 ✨
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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Will need a test case to show the issue
Did not test.