2 files changed, 15 insertions, 0 deletions
diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index fc6ac2877..200668f9e 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1045,6 +1045,8 @@ CustomElementReactionsStack::Enqueue(Element* aElement,
 
   // If the custom element reactions stack is empty, then:
   // Add element to the backup element queue.
+  MOZ_ASSERT(!aReaction->IsUpgradeReaction(),
+             "Upgrade reaction should not be scheduled to backup queue");
   mBackupQueue.AppendElement(aElement);
   elementData->mReactionQueue.AppendElement(aReaction);
 
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 674c653ab..867c92647 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -217,6 +217,16 @@ public:
   virtual void Traverse(nsCycleCollectionTraversalCallback& aCb) const
   {
   }
+
+#if DEBUG
+  bool IsUpgradeReaction()
+  {
+    return mIsUpgradeReaction;
+  }
+
+protected:
+  bool mIsUpgradeReaction = false;
+#endif
 };
 
 class CustomElementUpgradeReaction final : public CustomElementReaction
@@ -225,6 +235,9 @@ public:
   explicit CustomElementUpgradeReaction(CustomElementDefinition* aDefinition)
     : mDefinition(aDefinition)
   {
+#if DEBUG
+    mIsUpgradeReaction = true;
+#endif
   }
 
 private: