import changes from rmottola/Arctic-Fox:

- Bug 945584: Part 1 - Style support for scroll snapping attributes, - Implemented style support for new attributes:   - scroll-snap-type   - scroll-snap-type-x   - scroll-snap-type-y   - scroll-snap-points-x   - scroll-snap-points-y   - scroll-snap-destination   - scroll-snap-coordinate (02fe3bb49)
- Bug 945584: Part 2 - Add CSS scroll snapping attributes to ScrollbarStyles (v10 Patch) (7ffff307b)
- Bug 945584: Part 3 - Enable cancellation of OSX synthesized mousewheel scrolling events (v2 Patch), (124e19201)
- Bug 945584: Part 4 - Add scroll snapping preferences (v3 Patch), - Added preferences to allow trackpad and mousewheel flinging between snap points to be tuned: - layout.css.scroll-snap.prediction-max-velocity - layout.css.scroll-snap.prediction-sensitivity (64d44c6ca)
- Bug 945584: Part 5 - Implement ScrollVelocityQueue (v4 Patch), - Implemented ScrollVelocityQueue class to calculate the velocity of a scroll   when given periodic samples of scroll position. - Added BasePoint::Clamp to simplify code. (bca79b509)
- Inactive subframes-in-subframes add event regions to the wrong layer. (bug 1139213, r=tn) (c5ad5ede0)
- Bug 1075702 - Fixed implementation of Element.setAttributeNode(). (8e5cdb94d)
- Bug 1134968. If JS_NewObjectWithGivenProto is passed a null parent and null proto, use the context's current global as the parent. (82810e810)
- Bug 990907 - Add a flag on nsIScrollableFrame to indicate if it is actively being scrolled by APZ. (3eb29e60b)

dom/base/nsDOMAttributeMap.cpp

@@ -317,46 +317,51 @@ nsDOMAttributeMap::SetNamedItemInternal(Attr& aAttr,
   }
 
   // Get nodeinfo and preexisting attribute (if it exists)
-  nsAutoString name;
-  nsRefPtr<mozilla::dom::NodeInfo> ni;
+  nsRefPtr<NodeInfo> oldNi;
+
+  if (!aWithNS) {
+    nsAutoString name;
+    aAttr.GetName(name);
+    oldNi = mContent->GetExistingAttrNameFromQName(name);
+  }
+  else {
+    uint32_t i, count = mContent->GetAttrCount();
+    for (i = 0; i < count; ++i) {
+      const nsAttrName* name = mContent->GetAttrNameAt(i);
+      int32_t attrNS = name->NamespaceID();
+      nsIAtom* nameAtom = name->LocalName();
+
+      // we're purposefully ignoring the prefix.
+      if (aAttr.NodeInfo()->Equals(nameAtom, attrNS)) {
+        oldNi = mContent->NodeInfo()->NodeInfoManager()->
+          GetNodeInfo(nameAtom, name->GetPrefix(), aAttr.NodeInfo()->NamespaceID(),
+                      nsIDOMNode::ATTRIBUTE_NODE);
+        break;
+      }
+    }
+  }
 
   nsRefPtr<Attr> attr;
-  // SetNamedItemNS()
-  if (aWithNS) {
-    // Return existing attribute, if present
-    ni = aAttr.NodeInfo();
 
-    if (mContent->HasAttr(ni->NamespaceID(), ni->NameAtom())) {
-      attr = RemoveAttribute(ni);
+  if (oldNi) {
+    nsRefPtr<Attr> oldAttr = GetAttribute(oldNi, true);
+
+    if (oldAttr == &aAttr) {
+      return oldAttr.forget();
     }
-  } else { // SetNamedItem()
-    aAttr.GetName(name);
 
-    // get node-info of old attribute
-    ni = mContent->GetExistingAttrNameFromQName(name);
-    if (ni) {
-      attr = RemoveAttribute(ni);
-    }
-    else {
-      if (mContent->IsInHTMLDocument() &&
-          mContent->IsHTML()) {
-        nsContentUtils::ASCIIToLower(name);
-      }
-
-      rv = mContent->NodeInfo()->NodeInfoManager()->
-        GetNodeInfo(name, nullptr, kNameSpaceID_None,
-                    nsIDOMNode::ATTRIBUTE_NODE, getter_AddRefs(ni));
-      if (NS_FAILED(rv)) {
-        aError.Throw(rv);
-        return nullptr;
-      }
-      // value is already empty
+    if (oldAttr) {
+      attr = RemoveNamedItem(oldNi, aError);
+      NS_ASSERTION(attr->NodeInfo()->NameAndNamespaceEquals(oldNi),
+        "RemoveNamedItem() called, attr->NodeInfo() should be equal to oldNi!");
     }
   }
 
   nsAutoString value;
   aAttr.GetValue(value);
 
+  nsRefPtr<NodeInfo> ni = aAttr.NodeInfo();
+
   // Add the new attribute to the attribute map before updating
   // its value in the element. @see bug 364413.
   nsAttrKey attrkey(ni->NamespaceID(), ni->NameAtom());
@@ -374,6 +379,15 @@ nsDOMAttributeMap::SetNamedItemInternal(Attr& aAttr,
   return attr.forget();
 }
 
+already_AddRefed<Attr>
+nsDOMAttributeMap::RemoveNamedItem(NodeInfo* aNodeInfo, ErrorResult& aError)
+{
+  nsRefPtr<Attr> attribute = GetAttribute(aNodeInfo, true);
+  // This removes the attribute node from the attribute map.
+  aError = mContent->UnsetAttr(aNodeInfo->NamespaceID(), aNodeInfo->NameAtom(), true);
+  return attribute.forget();
+}
+
 NS_IMETHODIMP
 nsDOMAttributeMap::RemoveNamedItem(const nsAString& aName,
                                    nsIDOMAttr** aReturn)
@@ -399,11 +413,7 @@ nsDOMAttributeMap::RemoveNamedItem(const nsAString& aName, ErrorResult& aError)
     return nullptr;
   }
 
-  nsRefPtr<Attr> attribute = GetAttribute(ni, true);
-
-  // This removes the attribute node from the attribute map.
-  aError = mContent->UnsetAttr(ni->NamespaceID(), ni->NameAtom(), true);
-  return attribute.forget();
+  return RemoveNamedItem(ni, aError);
 }
 
 
@@ -501,6 +511,7 @@ nsDOMAttributeMap::GetAttrNodeInfo(const nsAString& aNamespaceURI,
     int32_t attrNS = name->NamespaceID();
     nsIAtom* nameAtom = name->LocalName();
 
+    // we're purposefully ignoring the prefix.
     if (nameSpaceID == attrNS &&
         nameAtom->Equals(aLocalName)) {
       nsRefPtr<mozilla::dom::NodeInfo> ni;
@@ -537,11 +548,7 @@ nsDOMAttributeMap::RemoveNamedItemNS(const nsAString& aNamespaceURI,
     return nullptr;
   }
 
-  nsRefPtr<Attr> attr = RemoveAttribute(ni);
-  mozilla::dom::NodeInfo* attrNi = attr->NodeInfo();
-  mContent->UnsetAttr(attrNi->NamespaceID(), attrNi->NameAtom(), true);
-
-  return attr.forget();
+  return RemoveNamedItem(ni, aError);
 }
 
 uint32_t