Sidebars appear to the right or left of the content when they fit, or collapse into a content header or footer on narrow screens.
Sidebars can be implemented as a region in the page template, configured in the block layout.
Sidebars could also be…
Sidebars
Form related classes.
Starterkit form.css
Default theme css.
/core/themes/starterkit_theme/css/components/form.css
.form-item.form-wrapper.form-actions
.label.description
.form-composite # Used in radios, checkboxes - see CompositeFormElementTrait
.…
Form API classes
To get this to appear, set your debugger to break on first line, and trigger an ajax load.
This was recently updated in core.
https://www.drupal.org/project/drupal/issues/1974928
https://www.drupal.org/node/…
Ajax loader