Automatic opening of collapsed filters with selections not working for single checkbox selection

Problem/Motivation

I have a multi-value-accepting facets exposed filter using a BEF checkboxes element with the "Make filter options collapsible" option checked. The facets exposed filter is configured to order any selected options above deselected options.

If I check one box and submit the form, BEF's automatic opening mechanism fails and the filter remains collapsed (as if no selections were made). If I expand the filter I can see my single selection.

Checking two boxes works and the filter remains expanded after form submission.

Steps to reproduce

See above.

Proposed resolution

I think we should modify this code in BetterExposedFiltersWidgetBase to work with checkboxes:

       // Check if one or more values are set for our current element.
      $options = $form[$element]['#options'] ?? [];
      $default_value = $form[$element]['#default_value'] ?? key($options);
      $has_values = array_reduce($user_input, function ($carry, $value) use ($form, $element, $default_value) {
        return $carry || ($value === $default_value ? '' : ($value || $default_value === 0));
      }, FALSE);

Currently, in the case of a checkboxes element with no pre-checked default (so no $form[$element]['#default_value']), this code will incorrectly assume the first checkbox option to be the empty/default option, and if checked, incorrectly conclude that the element has no values set.

I think this could also be a problem for a required radio element where the first option is not an empty option.

Remaining tasks

MR + review.

User interface changes

Automatic opening of collapsible BEF checkboxes elements with a single selection.

API changes

None.

Data model changes

None.