1 files changed, 40 insertions, 1 deletions
diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index ca00a49a5..d7e7a78f4 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -152,7 +152,7 @@ nsIContent::FindFirstNonChromeOnlyAccessContent() const
 }
 
 nsINode*
-nsIContent::GetFlattenedTreeParentNodeInternal() const
+nsIContent::GetFlattenedTreeParentNodeInternal(FlattenedParentType aType) const
 {
   nsINode* parentNode = GetParentNode();
   if (!parentNode || !parentNode->IsContent()) {
@@ -161,6 +161,45 @@ nsIContent::GetFlattenedTreeParentNodeInternal() const
   }
   nsIContent* parent = parentNode->AsContent();
 
+  if (aType == eForStyle &&
+      IsRootOfNativeAnonymousSubtree() &&
+      OwnerDoc()->GetRootElement() == parent) {
+    // When getting the flattened tree parent for style, we return null
+    // for any "document level" native anonymous content subtree root.
+    // This is NAC generated by an ancestor frame of the document element's
+    // primary frame, and includes scrollbar elements created by the root
+    // scroll frame, and the "custom content container" and accessible caret
+    // generated by the nsCanvasFrame.  We distinguish document level NAC
+    // from NAC generated by the root element's primary frame below.
+    nsIFrame* parentFrame = parent->GetPrimaryFrame();
+    if (!parentFrame) {
+      // If the root element has no primary frame, it means it can't have
+      // generated any NAC itself.  Thus any NAC we have here must have
+      // been generated by an ancestor frame.
+      //
+      // If we are in here, then either the root element is display:none, or
+      // we are in the middle of constructing the root of the frame tree and
+      // we are trying to eagerly restyle document level NAC in
+      // nsCSSFrameConstructor::GetAnonymousContent before the root
+      // element's frame has been constructed.
+      return nullptr;
+    }
+    nsIAnonymousContentCreator* creator = do_QueryFrame(parentFrame);
+    if (!creator) {
+      // If the root element does have a frame, but does not implement
+      // nsIAnonymousContentCreator, then this must be document level NAC.
+      return nullptr;
+    }
+    AutoTArray<nsIContent*, 8> elements;
+    creator->AppendAnonymousContentTo(elements, 0);
+    if (!elements.Contains(this)) {
+      // If the root element does have a frame, and also does implement
+      // nsIAnonymousContentCreator, but didn't create this node, then
+      // it must be document level NAC.
+      return nullptr;
+    }
+  }
+
   if (parent && nsContentUtils::HasDistributedChildren(parent) &&
       nsContentUtils::IsInSameAnonymousTree(parent, this)) {
     // This node is distributed to insertion points, thus we