Backed out as it made performance worse for some reason

layout/xul/base/src/nsMenuPopupFrame.cpp

@@ -254,30 +254,6 @@ nsMenuPopupFrame::SetInitialChildList(nsIAtom* aListName,
   return nsBoxFrame::SetInitialChildList(aListName, aChildList);
 }
 
-PRBool
-nsMenuPopupFrame::IsLeaf() const
-{
-  if (mGeneratedChildren)
-    return PR_FALSE;
-
-  // any popup with a type attribute, such as the autocomplete popup,
-  // is always generated right away.
-  if (mContent->HasAttr(kNameSpaceID_None, nsGkAtoms::type))
-    return PR_FALSE;
-
-  if (mPopupType != ePopupTypeMenu)
-    return PR_TRUE;
-
-  // menu popups generate their child frames lazily only when opened, so
-  // behave like a leaf frame. However, generate child frames normally if
-  // the parent menu has a sizetopopup attribute. In this case the size of
-  // the parent menu is dependant on the size of the popup, so the frames
-  // need to exist in order to calculate this size.
-  nsIContent* parentContent = mContent->GetParent();
-  return (parentContent &&
-          !parentContent->HasAttr(kNameSpaceID_None, nsGkAtoms::sizetopopup));
-}
-
 void
 nsMenuPopupFrame::AdjustView()
 {

layout/xul/base/src/nsMenuPopupFrame.h

@@ -183,7 +183,22 @@ public:
   NS_IMETHOD SetInitialChildList(nsIAtom*        aListName,
                                  nsIFrame*       aChildList);
 
-  virtual PRBool IsLeaf() const;
+  virtual PRBool IsLeaf() const
+  {
+    if (!mGeneratedChildren && mPopupType == ePopupTypeMenu) {
+      // menu popups generate their child frames lazily only when opened, so
+      // behave like a leaf frame. However, generate child frames normally if
+      // the parent menu has a sizetopopup attribute. In this case the size of
+      // the parent menu is dependant on the size of the popup, so the frames
+      // need to exist in order to calculate this size.
+      nsIContent* parentContent = mContent->GetParent();
+      if (parentContent &&
+          !parentContent->HasAttr(kNameSpaceID_None, nsGkAtoms::sizetopopup))
+        return PR_TRUE;
+    }
+
+    return PR_FALSE;
+  }
 
   // AdjustView should be called by the parent frame after the popup has been
   // laid out, so that the view can be shown.