src/View/Helper/ListFilterHelper.php
public function openContainer()
    {
        $classes = $this->config('containerClasses');

        $title = __d('list_filter', 'list_filter.filter_fieldset_title');

        if ($this->filterActive()) {
            $classes .= ' opened';
        } else {
            $classes .= ' closed';
        }

        $ret = $this->templater()->format('containerStart', [
            'attrs' => $this->templater()->formatAttributes([
                'class' => $classes
            ])
        ]);
        $ret .= $this->header();
        $ret .= $this->templater()->format('contentStart', [
            'attrs' => $this->templater()->formatAttributes([
                'class' => $this->config('contentClasses')
            ])
        ]);

        return $ret;
    }
public function openContainer()
    {
        $classes = $this->config('containerClasses');

        $title = __d('list_filter', 'list_filter.filter_fieldset_title');

        if ($this->filterActive()) {
            $classes .= ' opened';
        } else {
            $classes .= ' closed';
        }

        $ret = $this->templater()->format('containerStart', [
            'attrs' => $this->templater()->formatAttributes([
                'class' => $classes
            ])
        ]);
        $ret .= $this->header();
        $ret .= $this->templater()->format('contentStart', [
            'attrs' => $this->templater()->formatAttributes([
                'class' => $this->config('contentClasses')
            ])
        ]);

        return $ret;
    }
Opens the HTML container

@return HTML