Fix Ninja Forms Multi-Select option selection Issue. Prevent Default Selection & Enable Proper Multi-Select Handling in Forms.

fix-ninja-forms-multiselect-behavior.js

jQuery(document).ready(function($) {
    function fixMultiSelect() {
        $('select[multiple].ninja-forms-field:not([data-fixed])').each(function() {
            $(this).attr('data-fixed', 'true')
                .on('mousedown', function(e) {
                    e.preventDefault();
                    if (e.target.tagName === 'OPTION') {
                        var $opt = $(e.target);
                        $opt.prop('selected', !$opt.prop('selected'));
                        $(this).trigger('change').trigger('input');
                    }
                })
                .on('click', function(e) {
                    e.preventDefault();
                });
        });
    }

    var attempts = 0;
    var maxAttempts = 50;

    function waitAndFix() {
        attempts++;
        if ($('select[multiple].ninja-forms-field').length > 0) {
            fixMultiSelect();
        }
        if (attempts < maxAttempts) {
            setTimeout(waitAndFix, 200);
        }
    }

    waitAndFix();

    $(document).ajaxComplete(function() {
        setTimeout(fixMultiSelect, 100);
    });
});

Preview: