Mark hidden menu items on about:net-internals as disabled.

chrome/browser/resources/net_internals/tab_switcher_view.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Controller and View for switching between tabs.
  *
  * The View part of TabSwitcherView displays the contents of the currently
  * selected tab (only one tab can be active at a time).
  *
@@ skipping 75 matching lines @@
       view.show(false);
 
       // Add it to the dropdown menu.
       var menuItem = addNode(this.dropdownMenu_, 'option');
       menuItem.value = tabId;
       menuItem.textContent = name;
     },
 
     showMenuItem: function(tabId, isVisible) {
       var wasActive = this.activeTabId_ == tabId;
-      setNodeDisplay(this.getMenuItemNode_(tabId), isVisible);
+
+      // Hide the menuitem from the list. Note it needs to be 'disabled' to
+      // prevent it being selectable from keyboard.
+      var menuitem = this.getMenuItemNode_(tabId);
+      setNodeDisplay(menuitem, isVisible);
+      menuitem.disabled = !isVisible;
 
       if (wasActive && !isVisible) {
         // If the active tab is being hidden in the dropdown menu, then
         // switch to the first tab which is still visible.
         for (var i = 0; i < this.dropdownMenu_.options.length; ++i) {
           var option = this.dropdownMenu_.options[i];
           if (option.style.display != 'none') {
             this.switchToTab(option.value);
             break;
           }
@@ skipping 56 matching lines @@
     },
 
     onMenuSelectionChanged_: function(event) {
       var tabId = this.dropdownMenu_.value;
       this.switchToTab(tabId);
     },
   };
 
   return TabSwitcherView;
 })();