Display logic does not work if dispatcher is SearchableDropdownField

[sig-peggy]

The new SearchableDropdownField introduced in Silverstripe 5.2.0 does not work as the dispatcher for display logic. As 5.2.0 also uses this new field for auto scaffolding this change may break previously functional forms.

Example code

public function getCMSFields()
{
    $fields = parent::getCMSFields();
    $fields->addFieldsToTab('Root.Main', [
        SearchableDropdownField::create('Dispatcher', 'Select to show additional field', Page::get()),
        $testField = TextField::create('Hidden', 'Conditionally visible field')
    ]);
    $testField->displayIf('Dispatcher')->isNotEmpty();
    return $fields;
}

Changing the 'Dispatcher' selection will not reveal the 'Hidden' field in this example. Equally, if using displayUnless or hideIf, the 'Hidden' field would never be hidden. Using a DropdownField instead of a SearchableDropdownField works as expected.