Add some more comments...

IPython/html/static/tree/js/notebooklist.js

@@ -329,7 +329,14 b' define(['
         file: 'edit',
     };
 
+    /**
+     * Handles when any row selector checkbox is toggled.
+     */
     NotebookList.prototype._selection_changed = function() {
+
+        // Use a JQuery selector to find each row with a checked checkbox.  If
+        // we decide to add more checkboxes in the future, this code will need
+        // to be changed to distinguish which checkbox is the row selector.
         var selected = [];
         var has_running_notebook = false;
         var has_directory = false;
@@ -337,6 +344,8 b' define(['
         var that = this;
         $('.list_item :checked').each(function(index, item) {
             var parent = $(item).parent().parent();
+
+            // If the item doesn't have an upload button, it can be selected.
             if (parent.find('.upload_button').length === 0) {
                 selected.push({
                     name: parent.data('name'), 
@@ -344,6 +353,8 b' define(['
                     type: parent.data('type')
                 });
 
+                // Set flags according to what is selected.  Flags are later
+                // used to decide which action buttons are visible.
                 has_running_notebook = has_running_notebook || 
                     (parent.data('type') == 'notebook' && that.sessions[parent.data('path')] !== undefined);
                 has_file = has_file || parent.data('type') == 'file';
@@ -359,14 +370,15 b' define(['
             $('.rename-button').css('display', 'none');
         }
 
-        // Shutdown is only visible when one or more notebooks are visible.
+        // Shutdown is only visible when one or more notebooks running notebooks
+        // are selected and no non-notebook items are selected.
         if (has_running_notebook && !(has_file || has_directory)) {
             $('.shutdown-button').css('display', 'inline-block');
         } else {
             $('.shutdown-button').css('display', 'none');
         }
 
-        // Duplicate isn't visible if a directory is selected.
+        // Duplicate isn't visible when a directory is selected.
         if (selected.length > 0 && !has_directory) {
             $('.duplicate-button').css('display', 'inline-block');
         } else {