As determined in 📌 Improve entity display editing UI Needs work the form attaches the library `field_ui/drupal.field_ui'`. This library does some complex ajax logic we don't really needed.
Simplify things by dropping `field_ui/drupal.field_ui'` and adding a custom JS for what we really need, if anything.
What I see needed:
* AJAX reloads for handling the settings forms. That seems to be handled separately from `field_ui/drupal.field_ui'` via regular form ajax.
* Some logic to add/remove fields. We don't need regions. We only need to store region 'hidden' if disabled, and 'content' if enabled.
As UI for removing/hiding fields I think it would be much cleaner to drop the long-list of hidden fields. Instead, we should simply not render hidden fields but show a small select "Add fields", it lists hidden-fields and has a AJAX "Add field" button which reloads the form.
Then we need to have some alternative for removing a field. So some "X" button in the end, next to the settings button should be good enough there.
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Code
Agreed, and this is also necessary / a prerequisite to more easily support