show all category posts

functions.php

                function raindrops_category_id2name( $str ) {
                    $id = ( int ) $str;
                    return get_cat_name( $id );
                }

                function raindrops_reset_val( $str ) {
                    return 0;
                }

                function compare_capital_lower_not_distinguish( $a, $b ) {
                    return strcasecmp( $a, $b );
                }

                function raindrops_get_post_array_group_by_category( $limit_posts = 5, $args = array() ) {

                    global $post;

                    $category_ids = array_map( 'raindrops_category_id2name', get_all_category_ids() );
                    $category_ids = array_flip( $category_ids );
                    $category_ids = array_map( 'raindrops_reset_val', $category_ids );

                    if ( empty( $args ) ) {

                        query_posts( array( 'posts_per_page' => -1, 'post__status' => 'publish' ) );
                    } else {

                        query_posts( $args );
                    }

                    if ( have_posts() ) {

                        while ( have_posts() ) {

                            the_post();

                            $categories = get_the_category();
                            //var_dump( $categories );      

                            foreach ( $categories as $key => $val ) {

                                if ( empty( $result[$val -> name] ) || count( $result[$val -> name] ) < $limit_posts ) {

                                    $result[$val -> name][$post -> ID] = $post -> ID;
                                }
                            }
                        }
                    }
                    wp_reset_query();

                    uksort( $result, "compare_capital_lower_not_distinguish" );

                    return apply_filters( 'raindrops_get_post_array_group_by_category', $result );
                }

                function raindrops_display_recent_post_group_by_category( $limit_posts = 5, $args = array() ) {

                    $raindrops_get_post_array_group_by_category = raindrops_get_post_array_group_by_category( $limit_posts, $args );

                    $raindrops_date_format = get_option( 'date_format' ) . ' ' . get_option( 'time_format' );
                  //  $time_elements         = raindrops_doctype_elements( 'abbr', 'time', false );
                    $result                = apply_filters( 'raindrops_display_recent_post_group_by_category_before', '' );
                    $wrap_html             = '<ul class="xoxo">%1$s</ul>';
                    $category_title        = '<li><h3 class="category-title"><a href="%1$s">%2$s</a></h3><ul>';
                    $entry_item            = '<li><a href="%1$s">%3$s</a><p><span title="%4$s">%2$s</span> </p>';
                    $loop_end_html         = '</li></ul></li>';


                    foreach ( $raindrops_get_post_array_group_by_category as $key => $vals ) {

                        $cat_id = get_cat_ID( $key );

                        if ( !empty( $vals ) ) {

                            $result .= sprintf( $category_title, get_category_link( $cat_id ), $key );
                        }

                        foreach ( $vals as $val ) {
                            $permalink = esc_url( get_permalink( $val ) );
                            $date      = get_the_time( $raindrops_date_format, $val );
                            $title     = get_the_title( $val );
                            $result .= sprintf( $entry_item, $permalink, $date, $title, $date );
                        }
                        $result .= $loop_end_html;
                    }

                    $result = sprintf( $wrap_html, $result );
                    return apply_filters( 'raindrops_display_recent_post_group_by_category', $result );
                }

Bug fix

@see http://ja.forums.wordpress.org/topic/125267

Thanks @kjmtsh