Going through htmlform or whatever you use 'hide-if' for a field to hide it when something is selected/checked/whatever. Expected behaviour is for this to hide it.

Example in getFormFields():

$fields['blah'] = [
	'type' => 'text',
	'hide-if' => [ '===', 'previousthing', 'new' ]
];

This should hide it. But it doesn't.

This is currently one of the things blocking the conversion of the login form to ooui, but also just plain problematic in general because it's used a lot of other places too.