[gallery] shortcode accepts the
columns='0' as an attribute so that it doesn't insert any breaks in the HTML between gallery items, however this option is not available in Gallery Settings interface ("Add Media" pop-up when editing pages/posts).
I've located the code in
wp-includes\media-template.php line 396 to 406;
<label class="setting"> <span><?php _e('Columns'); ?></span> <select class="columns" name="columns" data-setting="columns"> <?php for ( $i = 1; $i <= 9; $i++ ) : ?> <option value="<?php echo esc_attr( $i ); ?>" <?php selected( $i, 3 ); ?>> <?php echo esc_html( $i ); ?> </option> <?php endfor; ?> </select> </label>
I don't want to directly edit a core file but I would like to add
<option value="0">Responsive</option> to that list. What's the best way to do that without editing core files? I was thinking some kind of filter, although I'm not too familiar with them.