Bug 946502. Part 5: A ViewportFrame with a displayport on the root element needs to be an animated geometry root. r=mattwoodrow

--HG--
extra : rebase_source : 15005e1ef96836deed4f6910a113be5f8a0b3316

layout/base/nsLayoutUtils.cpp

@@ -1315,6 +1315,11 @@ nsLayoutUtils::GetAnimatedGeometryRootFor(nsIFrame* aFrame,
       break;
     if (ActiveLayerTracker::IsOffsetOrMarginStyleAnimated(f))
       break;
+    if (!f->GetParent() && ViewportHasDisplayPort(f->PresContext())) {
+      // Viewport frames in a display port need to be animated geometry roots
+      // for background-attachment:fixed elements.
+      break;
+    }
     nsIFrame* parent = GetCrossDocParentFrame(f);
     if (!parent)
       break;