Add dropdown list relies on buttons being input elements

Problem/Motivation

The views_ui admin JavaScript requires that buttons in the interface be input elements when it is acceptable to change the markup into button elements in other places in Drupal.

The selector input.add-display is unnecessarily too strict. the add-display class is only used in views_ui module in Drupal core.

This would be a nice-to-have for people that like button elements, but it is a very minor use case because of the recent trend to direct people to use gin theme only.

Steps to reproduce

1. Generate a starterkit custom theme
2. Copy templates/form/input.html.twig to templates/form/input--submit.html.twig
3. Change the markup to <button{{ attributes.setAttribute('type', 'submit') }}>{{ element['#value'] }}</button>{{ children }}, for example.
4. Enable the theme as an administrative theme
5. Edit a view
6. The dropdown menu generated by views_ui/views_ui.admin library in js/views-admin.js does not generate any items.

Proposed resolution

There is already a pattern to handle something like this in views_ui module in the js/ajax.js file targeting multiple possible selectors - $form.find('input[type=submit].js-form-submit, button.js-form-submit').

We could add button.add-display as a possible selector in addition to input.add-display.

Remaining tasks

To be determined (TBD)

User interface changes

No

Introduced terminology

No

API changes

No

Data model changes

No

Release notes snippet