Bug 1343937 - Exempt scrollbar NAC from the new NAC semantics. r=bz

MozReview-Commit-ID: 8TbFkirYOy2

dom/base/nsIContent.h

@@ -959,6 +959,14 @@ public:
     return false;
   }
 
+  // Returns true if this element is native-anonymous scrollbar content.
+  bool IsNativeScrollbarContent() const {
+    return IsNativeAnonymous() &&
+           IsAnyOfXULElements(nsGkAtoms::scrollbar,
+                              nsGkAtoms::resizer,
+                              nsGkAtoms::scrollcorner);
+  }
+
   // Overloaded from nsINode
   virtual already_AddRefed<nsIURI> GetBaseURI(bool aTryUseXHRDocBaseURI = false) const override;
 

layout/base/nsCSSFrameConstructor.cpp

@@ -10858,6 +10858,12 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
     // first element that is not itself NAC (distinct from whether it happens
     // to be in a NAC subtree).
     //
+    // The one exception to all of this is scrollbar content, which we parent
+    // directly to the scrollframe. This is because the special-snowflake
+    // construction of scroll frames doesn't result in the placeholder frame
+    // being constructed until later, which means that GetInFlowParent() doesn't
+    // work right in the case of out-of-flow scrollframes.
+    //
     // To implement all this, we need to pass the correct parent style context
     // here because SetPrimaryFrame() may not have been called on the content
     // yet and thus ResolveStyleContext can't find it otherwise.
@@ -10867,9 +10873,12 @@ nsCSSFrameConstructor::AddFCItemsForAnonymousContent(
     // to worry about anonymous boxes, which CorrectStyleParentFrame handles
     // for us.
     nsIFrame* inheritFrame = aFrame;
-    while (inheritFrame->GetContent()->IsNativeAnonymous()) {
-      inheritFrame = inheritFrame->GetInFlowParent();
+    if (!content->IsNativeScrollbarContent()) {
+      while (inheritFrame->GetContent()->IsNativeAnonymous()) {
+        inheritFrame = inheritFrame->GetInFlowParent();
+      }
     }
+
     if (inheritFrame->GetType() == nsGkAtoms::canvasFrame) {
       // CorrectStyleParentFrame returns nullptr if the prospective parent is
       // the canvas frame, so avoid calling it in that situation.

layout/generic/nsFrame.cpp

@@ -9157,7 +9157,7 @@ GetCorrectedParent(const nsIFrame* aFrame)
   nsIContent* content = aFrame->GetContent();
   Element* element =
     content && content->IsElement() ? content->AsElement() : nullptr;
-  if (element && element->IsNativeAnonymous() &&
+  if (element && element->IsNativeAnonymous() && !element->IsNativeScrollbarContent() &&
       element->GetPseudoElementType() == aFrame->StyleContext()->GetPseudoType()) {
     while (parent->GetContent() && parent->GetContent()->IsNativeAnonymous()) {
       parent = parent->GetInFlowParent();