field (css class) defaults+

[rudiedirkx]

class_append didn't work for field types with specific config/classes.

Config:

return [
	'defaults' => [
		'field_class'         => 'form-control',

		'checkbox' => [
			'field_class' 		=> 'form-check-input',
		],
	],
];

Add field:

$this->add('mycheckbox', 'checkbox', [
	'label' => "Wadever",
	'attr' => ['class_append' => 'custom-checkbox'],
]);

That <input> would have class="form-check-input", not form-check-input custom-checkbox, because setDefaultClasses() (with form-check-input) happened after prepareOptions() (with class_append/custom-checkbox), so the custom classes was added and then immediately overwritten.