Only scroll on main if the targeted frames need to scroll on main

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index f3496595a49b91242f2929265d357eb26dab8317..2a817d90470e1d85b74227346d9c63e3aebe21b1 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -64,6 +64,7 @@ class DocumentLifecycle;
 class Cursor;
 class Element;
 class ElementVisibilityObserver;
+class Frame;
 class FloatSize;
 class JSONArray;
 class JSONObject;
@@ -767,6 +768,24 @@ class CORE_EXPORT FrameView final
   // Only for SPv2.
   std::unique_ptr<JSONObject> compositedLayersAsJSON(LayerTreeFlags);
 
+  // Recursively update frame tree. Each frame has its only

[pdr., 2016/12/17 05:50:16]

+1, nice comment.

[yigu, 2016/12/18 17:57:11]

Thanks for your previous suggestion on this :)

+  // scroll on main reason. Given the following frame tree
+  //   A
+  //  / \
+  // B   C
+  // |
+  // D
+  // If B has fixed background-attachment but other frames
+  // don't, both A and C should scroll on cc. Frame D should
+  // scrolled on main thread as its ancestor B.
+  void updateSubFrameScrollOnMainReason(const Frame&,
+                                        MainThreadScrollingReasons);
+  String mainThreadScrollingReasonsAsText() const;
+  MainThreadScrollingReasons mainThreadScrollingReasons() const;
+  MainThreadScrollingReasons mainThreadScrollingReasonsPerFrame(
+      const LocalFrame&) const;
+  bool hasVisibleSlowRepaintViewportConstrainedObjects(const FrameView&) const;
+
  protected:
   // Scroll the content via the compositor.
   bool scrollContentsFastPath(const IntSize& scrollDelta);
@@ -1132,6 +1151,8 @@ class CORE_EXPORT FrameView final
   // For Slimming Paint v2 only.
   std::unique_ptr<PaintController> m_paintController;
   std::unique_ptr<PaintArtifactCompositor> m_paintArtifactCompositor;
+
+  MainThreadScrollingReasons m_lastMainThreadScrollingReasons;

[pdr., 2016/12/17 05:50:16]

Is "last" important? m_mainThreadScrollingReasons may be simpler.

[yigu, 2016/12/18 17:57:11]

Done.

 };
 
 inline void FrameView::incrementVisuallyNonEmptyCharacterCount(unsigned count) {