Menu is disappears on "Admin | Content Mgmnt | Content" page

I had a similar problem using a version of garland that I've customized, and as you said, only on the content management page. Perhaps it affects the regular garland theme as well. Basically, it moved my right sidebar down to the bottom left corner of the page so that it was invisible unless you scroll all the way to the bottom. I decided that the problem likely had to do with the extra horizontal space required by the weight module settings. So I went to the module/weight/weight.module file and commented out lines 179-219 which provide the ability to manage the weight setting from the content management page. (I commented them out by putting /** at the beginning of 179 and */ at line 219 - see below.) That did the trick and pulled the right sidebar back into its original position on the content management page (something I hadn't managed to do with all the things I tried to do in styles.css). I like having the weight module, but I prefer having the menu accessible on the content management page - I'll just control the weight on the individual pages. Hope that helps someone solve the problem.

/**if ( $form_id == 'node_filter_form' ) {
    // no comment
    unset($form['filters']['status']['status']['#options']['sticky-1']);
    unset($form['filters']['status']['status']['#options']['sticky-0']);
    $form['weight_help'] = array(
      '#type'  => 'markup',
      '#value' => t('<strong>Note:</strong> When the weight module is enabled,
        it is not possible to filter based on sticky status.')
    );
  }

  if ( $form_id == 'node_admin_nodes' ) {

    // i can't add a table header for weight, so instead i'm going to explain
    // the weight dropdown to the user. Also, to position my help text
    // appropriately, i'm using this '#suffix' hack rather than adding
    // a form property as i'd like to do.
    $form['options']['#suffix'] = t('
      <strong>Weight:</strong> To change the weight of a node, select a value
      from the corresponding dropdown box under <i>@operations</i>. Node weights
      are submitted immediately. Selectors are only available for node types
      configured on the <a href="@weight_admin">weight admin page</a>.
      ', array(
                '@weight_admin' => url('admin/settings/weight'),
                '@operations'   => 'Operations'
              )
    );

    // add my weight selector under the operations section of the admin node
    // overview page (admin/content/node)
    if ( !empty($form['operations']) ) {
      foreach ( $form['operations'] as $nid => $title ) {
        // only add weight selector if weight is enabled for this node type
				// In Drupal5, name value is uppercase all of a sudden :(
        if ( in_array(strtolower($form['name'][$nid]['#value']), $weight_node_types) ) {
          $form['operations'][$nid]['weight_selector']['#value'] =  weight_node_selector($nid);
        }
      }
    }
  }
*/

I believe this is a duplicate of #159942: unclosed form?, which will be fixed soon.