WP3.6 custom fields dropdown with strange behaviour after update

I found a solution to this – or more likely a workaround. Append this to your theme’s functions.php and adjust the section “Add your options” accordingly to fit your needs:

/**
 * Programatically add custom fields.
 *
 * @see http://wordpress.stackexchange.com/questions/98269/programatically-add-options-to-add-new-custom-field-dropdown/
 */

function wpse_98269_script() {

    if (isset($GLOBALS['post'])) {

        $post_type = get_post_type($GLOBALS['post']);

        if (post_type_supports($post_type, 'custom-fields')) {

            ?>

                <script>

                    // Cache:
                    var $metakeyinput = jQuery('#metakeyinput'),
                        $metakeyselect = jQuery('#metakeyselect');

                    // Does the default input field exist and is it visible?
                    if ($metakeyinput.length && ( ! $metakeyinput.hasClass('hide-if-js'))) {

                        // Hide it:
                        $metakeyinput.addClass('hide-if-js'); // Using WP admin class.

                        // ... and create the select box:
                        $metakeyselect = jQuery('<select id="metakeyselect" name="metakeyselect">').appendTo('#newmetaleft');

                        // Add the default select value:
                        $metakeyselect.append('<option value="#NONE#">— Select —</option>');

                    }

                    // Add your options:
                    $metakeyselect.append("<option value='Your_option_1'>Your option 1</option>");
                    $metakeyselect.append("<option value='Your_option_2'>Your option 2</option>");
                    $metakeyselect.append("<option value='Your_option_3'>Your option 3</option>");
                    $metakeyselect.append("<option value='Your_option_4'>Your option 4</option>");
                    $metakeyselect.append("<option value='Your_option_5'>Your option 5</option>");
                    $metakeyselect.append("<option value='Your_option_6'>Your option 6</option>");

                </script>

            <?php

        }

    }

}

add_action('admin_footer-post-new.php', 'wpse_98269_script');
add_action('admin_footer-post.php', 'wpse_98269_script');

filtering the list or simply resetting it to no content at all can be easily done with jquery…

BTW: If I got my reading right, this is *not a bug* but intended behaviour as of WP 3.6

Just add the following right above the line “// Add your options:”

jQuery('#metakeyselect').find('option').remove();
$metakeyselect.append('<option value="#NONE#">— Select —</option>');

But seriously – google is your friend here. Consider yourself lucky that I am in a great mood today, since you can not expect other people to do all the work for you…