When a dialog contains a form, that form is submitted and there are validation errors, the form (in the error state including the error messages) is rendered outside of the dialog. This is unintuitive and confusing.
The problem is that the submit button is (generally) not an Ajax-button, so that it actually sends a POST request just like it would on its own page and, thus, Drupal behaves exactly like that. It's just that if there are no errors the expected result of the "within dialog" form and the "without dialog" form are the same: return to the page which lead to the form / opening of the dialog.
Expected result:
The form error is shown inside of the dialog
Actual result:
The dialog is closed and the form error is shown on its own page leaving the context of the block library (but for the breadcrumb).
See also the attached screen recording.
Not really sure. I tried on a custom form to Ajax-ify the submit button, but this lead me down starting to re-implement the entire form rebuilding/validation process as Ajax commands until I stopped and opened this issue. Basically for the case of submitting from a dialog we somehow need to adapt \Drupal\Core\Form\FormSubmitter::doSubmitForm() to not issue a redirect but instead rebuild the form and return an Ajax response to replace the form within the dialog with the rebuilt form. And then the submit buttons would need to be automatically Ajax-ified to avoid the POST request. Maybe someone else has a less invasive idea. 🤷
Form errors are properly rendered within a dialog as per the above.
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Active
11.0 🔥
forms system
As far as I can tell this was worked around in ✨ Use modal in add new field flow Active (for that particular case) but have not delved into it in depth if that is in fact the case and, if so, if that be generalized in some way.