2 files changed, 31 insertions, 1 deletions
diff --git a/layout/base/RestyleManager.cpp b/layout/base/RestyleManager.cpp
index c3eeb8c71..16db4c18b 100644
--- a/layout/base/RestyleManager.cpp
+++ b/layout/base/RestyleManager.cpp
@@ -1062,6 +1062,25 @@ ElementForStyleContext(nsIContent* aParentContent,
     return f->GetContent()->AsElement();
   }
 
+  Element* frameElement = aFrame->GetContent()->AsElement();
+  if (frameElement->IsNativeAnonymous() &&
+      nsCSSPseudoElements::PseudoElementIsJSCreatedNAC(aPseudoType)) {
+    // NAC-implemented pseudos use the closest non-NAC element as their
+    // element to inherit from.
+    //
+    // FIXME(heycam): In theory we shouldn't need to limit this only to
+    // JS-created pseudo-implementing NAC, as all pseudo-implementing
+    // should use the closest non-native anonymous ancestor element as
+    // its originating element.  But removing that part of the condition
+    // reveals some bugs in style resultion with display:contents and
+    // XBL.  See bug 1345809.
+    Element* originatingElement =
+      nsContentUtils::GetClosestNonNativeAnonymousAncestor(frameElement);
+    if (originatingElement) {
+      return originatingElement;
+    }
+  }
+
   if (aParentContent) {
     return aParentContent->AsElement();
   }
diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index 3be7c2a0b..b574e7690 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -5041,8 +5041,19 @@ nsCSSFrameConstructor::ResolveStyleContext(nsStyleContext* aParentStyleContext,
                                            aParentStyleContext);
       }
     } else {
-      MOZ_ASSERT(aOriginatingElementOrNull);
       MOZ_ASSERT(aContent->IsInNativeAnonymousSubtree());
+      if (!aOriginatingElementOrNull) {
+        // For pseudo-implementing NAC created by JS using the ChromeOnly
+        // document.createElement(..., { pseudo: ... }) API, we find the
+        // originating element by lookup the tree until we find a non-NAC
+        // ancestor.  (These are the correct semantics for C++-generated pseudo-
+        // implementing NAC as well, but for those cases we already have a
+        // correct originating element passed in.)
+        MOZ_ASSERT(nsCSSPseudoElements::PseudoElementIsJSCreatedNAC(pseudoType));
+        aOriginatingElementOrNull =
+          nsContentUtils::GetClosestNonNativeAnonymousAncestor(aContent->AsElement());
+      }
+      MOZ_ASSERT(aOriginatingElementOrNull);
       result = styleSet->ResolvePseudoElementStyle(aOriginatingElementOrNull,
                                                    pseudoType,
                                                    aParentStyleContext,