ATK4 Form datePicker - default state is 'opened', can this be changed?

I'm using the following form, and everytime the page opens (it's in an expander on a grid) the datePicker is 'open', obscuring part of the text above it.

function page_del() {
    $billingid  = $_GET['id'];
    $now        = date("Y-m-d H:i:s"); 

    $q = $this->api->db->dsql()
        ->table('billing')
        ->where('id', $billingid)
        ->field('enddate')
        ->getOne();

    if (!$q) {
        $this->add('H5')->set('Are you sure you want to stop billing this item?');
        $form = $this->add('Form');
        $form->addField('hidden','billingid')->set($billingid);
        $form->addField('datePicker','datum')->set($now);
        $form->addSubmit('Confirm');

        if ($form->isSubmitted()) {
            $form->stopBilling('manual', $form, $now);              
            $this->js()->univ()->getjQuery()->trigger('reload_grid')->execute(); 
        }
    } else {
            $this->add('H5')->set('This product has already been stopped, effective date: ' .$q);
            }
        }
}

I have other forms elsewhere that also have a datePicker as their first (visible) field that do not display this behaviour. I only mention it because it looks like a 'focus' issue? I.e. the first field gets focus?

Any thoughts on what causes this or how it can be remedied?

Actually it is field state "onfocus", not default. Your form has only one field and this (first) field is selected on page load.

This behavior is added here:

https://github.com/atk4/atk4/blob/master/lib/Form/Field/DatePicker.php#L35

function addCalendarIcon() {
    $this->addButton('',array('options'=>array('text'=>false)))
        ->setHtml(' ')
        ->setIcon('ui-icon-calendar')
        ->js('click',$this->js()->datepicker('show'));
    $this->js('focus', $this->js()->datepicker('show'));
}

You can redefine this method in your project and remove line

$this->js('focus', $this->js()->datepicker('show'));