diff --git a/layout/xul/nsMenuPopupFrame.cpp b/layout/xul/nsMenuPopupFrame.cpp
index 034cd031f034..06e0d22141fb 100644
--- a/layout/xul/nsMenuPopupFrame.cpp
+++ b/layout/xul/nsMenuPopupFrame.cpp
@@ -58,7 +58,6 @@
 
 using namespace mozilla;
 using mozilla::dom::Event;
-using mozilla::dom::Document;
 using mozilla::dom::KeyboardEvent;
 
 int8_t nsMenuPopupFrame::sDefaultLevelIsTop = -1;
@@ -392,6 +391,15 @@ void nsXULPopupShownEvent::CancelListener() {
 NS_IMPL_ISUPPORTS_INHERITED(nsXULPopupShownEvent, Runnable,
                             nsIDOMEventListener);
 
+void nsMenuPopupFrame::SetInitialChildList(ChildListID aListID,
+                                           nsFrameList& aChildList) {
+  // unless the list is empty, indicate that children have been generated.
+  if (aListID == kPrincipalList && aChildList.NotEmpty()) {
+    mGeneratedChildren = true;
+  }
+  nsBoxFrame::SetInitialChildList(aListID, aChildList);
+}
+
 bool nsMenuPopupFrame::IsLeafDynamic() const {
   if (mGeneratedChildren) return false;
 
diff --git a/layout/xul/nsMenuPopupFrame.h b/layout/xul/nsMenuPopupFrame.h
index 42746476428f..dd288e7f8c33 100644
--- a/layout/xul/nsMenuPopupFrame.h
+++ b/layout/xul/nsMenuPopupFrame.h
@@ -247,6 +247,9 @@ class nsMenuPopupFrame final : public nsBoxFrame,
   nsresult CreateWidgetForView(nsView* aView);
   uint8_t GetShadowStyle();
 
+  virtual void SetInitialChildList(ChildListID aListID,
+                                   nsFrameList& aChildList) override;
+
   virtual bool IsLeafDynamic() const override;
 
   virtual void UpdateWidgetProperties() override;
diff --git a/layout/xul/nsXULPopupManager.cpp b/layout/xul/nsXULPopupManager.cpp
index 9e70f6d1f42f..56506fcfb80d 100644
--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -1284,11 +1284,8 @@ void nsXULPopupManager::FirePopupShowingEvent(nsIContent* aPopup,
   nsPopupType popupType = popupFrame->PopupType();
 
   // generate the child frames if they have not already been generated
-  const bool generateFrames = popupFrame->IsLeaf();
-  MOZ_ASSERT_IF(generateFrames, !popupFrame->HasGeneratedChildren());
-  popupFrame->SetGeneratedChildren();
-  if (generateFrames) {
-    MOZ_ASSERT(popupFrame->PrincipalChildList().IsEmpty());
+  if (!popupFrame->HasGeneratedChildren()) {
+    popupFrame->SetGeneratedChildren();
     presShell->FrameConstructor()->GenerateChildFrames(popupFrame);
   }