Adding some comments.

layout/xul/base/src/nsMenuBarFrame.cpp

@@ -30,6 +30,14 @@
 #include "nsIDOMEventReceiver.h"
 #include "nsXULAtoms.h"
 
+// XXX Notes to self
+// 1. The menu bar should get its own view, and it should capture all events when the ALT key
+// goes down.
+//
+// 2. Collapsing the menu bar will destroy the frame. The menu bar will need to unregister its
+// listeners when this happens.
+//
+
 //
 // NS_NewMenuBarFrame
 //
@@ -139,15 +147,7 @@ nsMenuBarFrame::KeyboardNavigation(PRUint32 aDirection)
         GetNextMenuItem(mCurrentMenu, getter_AddRefs(nextItem));
       else GetPreviousMenuItem(mCurrentMenu, getter_AddRefs(nextItem));
 
-      if (mCurrentMenu == nextItem.get())
-        return;
-
-      // Unset the current child.
-      mCurrentMenu->UnsetAttribute(kNameSpaceID_None, nsXULAtoms::menuactive, PR_TRUE);
-      
-      // Set the new child.
-      nextItem->SetAttribute(kNameSpaceID_None, nsXULAtoms::menuactive, "true", PR_TRUE);
-      mCurrentMenu = nextItem;
+      SetCurrentMenuItem(nextItem);
     }
   }
 }
@@ -251,3 +251,16 @@ nsMenuBarFrame::GetPreviousMenuItem(nsIContent* aStart, nsIContent** aResult)
   NS_IF_ADDREF(aStart);
 }
 
+void nsMenuBarFrame::SetCurrentMenuItem(nsIContent* aMenuItem)
+{
+  if (mCurrentMenu == aMenuItem)
+    return;
+
+  // Unset the current child.
+  mCurrentMenu->UnsetAttribute(kNameSpaceID_None, nsXULAtoms::menuactive, PR_TRUE);
+  
+  // Set the new child.
+  aMenuItem->SetAttribute(kNameSpaceID_None, nsXULAtoms::menuactive, "true", PR_TRUE);
+  mCurrentMenu = aMenuItem;
+}
+

layout/xul/base/src/nsMenuBarFrame.h

@@ -51,6 +51,8 @@ public:
   void GetPreviousMenuItem(nsIContent* aStart, nsIContent** aResult);
   void KeyboardNavigation(PRUint32 aDirection);
 
+  void SetCurrentMenuItem(nsIContent* aMenuItem);
+
 protected:
   nsMenuBarListener* mMenuBarListener; // The listener that tells us about key and mouse events.
   PRBool mIsActive; // Whether or not the menu bar is active (a menu item is highlighted or shown).

layout/xul/base/src/nsMenuBarListener.cpp

@@ -133,6 +133,33 @@ nsMenuBarListener::HandleEvent(nsIDOMEvent* aEvent)
 nsresult
 nsMenuBarListener::MouseMove(nsIDOMEvent* aMouseEvent)
 {
+  if (mMenuBarFrame->IsActive()) {
+    // Make sure the active menu gets set properly.
+    // If we're over something then switch to it.
+    nsCOMPtr<nsIDOMNode> target;
+    aMouseEvent->GetTarget(getter_AddRefs(target));
+
+    nsCOMPtr<nsIContent> current = do_QueryInterface(target);
+    nsCOMPtr<nsIContent> menuBar;
+    mMenuBarFrame->GetContent(getter_AddRefs(menuBar));
+
+    while (current && (current.get() != menuBar)) {
+      // See if we're a menu item.
+      nsCOMPtr<nsIAtom> tag;
+      current->GetTag(*getter_AddRefs(tag));
+      if (tag && (tag.get() == nsXULAtoms::xpmenu)) {
+        // We found a menu.
+        mMenuBarFrame->SetCurrentMenuItem(current);
+        break;
+      }
+
+      // Get our parent.
+      nsCOMPtr<nsIContent> parent;
+      current->GetParent(*getter_AddRefs(parent));
+      current = parent.get();
+    }
+  }
+
   return NS_OK; // means I am NOT consuming event
 }