Display Posts Under Categories in Sidebar

Hi all,
This is proving more difficult that I had thought. Currently only the categories in my portfolio sidebar are shown but I would like to click on each category menu item and have all posts titles within it appear below, with only one category allowed ‘open’ at a time. Additionally, I would like this same sidebar to be displayed as you navigate from portfolio landing page to all posts within a specific category to a single portfolio post.

Category A
Category B
Post 1
Post 2
Post 3
Category C

Existing code for sidebar in Portfolio Page Template (template-portfolio.php)

<!--BEGIN .sidebar -->
            	<div class="sidebar">
                    <?php the_content(); ?>
                     <ul id="filter">
                      <?php wp_list_categories(array('title_li' => '', 'taxonomy' => 'skill-type', 'walker' => new Portfolio_Walker())); ?>		

                <!--END .sidebar -->

Existing code in functions.php:

class Portfolio_Walker extends Walker_Category {
   function start_el(&$output, $category, $depth, $args) {
      extract($args);
      $cat_name = esc_attr( $category->name);
      $cat_name = apply_filters( 'list_cats', $cat_name, $category );
	  $link = '<a href="#">term_id.'" ';
      if ( $use_desc_for_title == 0 || empty($category->description) )
         $link .= 'title="' . sprintf(__( 'View all items filed under %s' ), $cat_name) . '"';
      else
         $link .= 'title="' . esc_attr( strip_tags( apply_filters( 'category_description', $category->description, $category ) ) ) . '"';
      $link .= '>';
      // $link .= $cat_name . '</a>';
      $link .= $cat_name;
      if(!empty($category->description)) {
         $link .= ' <span>'.$category->description.'</span>';
      }
      $link .= '';
      if ( (! empty($feed_image)) || (! empty($feed)) ) {
         $link .= ' ';
         if ( empty($feed_image) )
            $link .= '(';
         $link .= '<a>term_id, $feed_type) . '"';
         if ( empty($feed) )
            $alt = ' alt="' . sprintf(__( 'Feed for all posts filed under %s' ), $cat_name ) . '"';
         else {
            $title = ' title="' . $feed . '"';
            $alt = ' alt="' . $feed . '"';
            $name = $feed;
            $link .= $title;
         }
         $link .= '>';
         if ( empty($feed_image) )
            $link .= $name;
         else
            $link .= "<img src='$feed_image'$alt$title" . ' />';
         $link .= '</a>';
         if ( empty($feed_image) )
            $link .= ')';
      }
      if ( isset($show_count) && $show_count )
         $link .= ' (' . intval($category->count) . ')';
      if ( isset($show_date) && $show_date ) {
         $link .= ' ' . gmdate('Y-m-d', $category->last_update_timestamp);
      }
      if ( isset($current_category) && $current_category )
         $_current_category = get_category( $current_category );
      if ( 'list' == $args['style'] ) {
          $output .= '<li class="segment-'.rand(2, 99).'"';
          $class = 'cat-item cat-item-'.$category->term_id;
          if ( isset($current_category) && $current_category && ($category->term_id ==

$current_category) )
$class .= ‘ current-cat’;
elseif ( isset($_current_category) && $_current_category && ($category->term_id == $_current_category->parent) )
$class .= ‘ current-cat-parent’;
$output .= ”;
$output .= “>$link\n”;
} else {
$output .= “\t$link\n”;
}
}
}`

Thanks!