1 files changed, 24 insertions, 0 deletions
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 9ced64c0d..092755590 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -183,6 +183,12 @@ Element::DoGetClasses() const
 NS_IMETHODIMP
 Element::QueryInterface(REFNSIID aIID, void** aInstancePtr)
 {
+  if (aIID.Equals(NS_GET_IID(Element))) {
+    NS_ADDREF_THIS();
+    *aInstancePtr = this;
+    return NS_OK;
+  }
+
   NS_ASSERTION(aInstancePtr,
                "QueryInterface requires a non-NULL destination!");
   nsresult rv = FragmentOrElement::QueryInterface(aIID, aInstancePtr);
@@ -1838,6 +1844,24 @@ Element::UnbindFromTree(bool aDeep, bool aNullParent)
     SetParentIsContent(false);
   }
 
+#ifdef DEBUG
+  // If we can get access to the PresContext, then we sanity-check that
+  // we're not leaving behind a pointer to ourselves as the PresContext's
+  // cached provider of the viewport's scrollbar styles.
+  if (document) {
+    nsIPresShell* presShell = document->GetShell();
+    if (presShell) {
+      nsPresContext* presContext = presShell->GetPresContext();
+      if (presContext) {
+        MOZ_ASSERT(this !=
+                   presContext->GetViewportScrollbarStylesOverrideNode(),
+                   "Leaving behind a raw pointer to this node (as having "
+                   "propagated scrollbar styles) - that's dangerous...");
+      }
+    }
+  }
+#endif
+
   // Ensure that CSS transitions don't continue on an element at a
   // different place in the tree (even if reinserted before next
   // animation refresh).