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

third_party/WebKit/Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All
  * Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 24 matching lines @@
 #include "core/frame/FrameConsole.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/RemoteFrame.h"
 #include "core/frame/RemoteFrameView.h"
 #include "core/frame/Settings.h"
 #include "core/frame/VisualViewport.h"
 #include "core/html/HTMLMediaElement.h"
 #include "core/inspector/ConsoleMessageStorage.h"
 #include "core/inspector/InspectorInstrumentation.h"
+#include "core/layout/LayoutView.h"
 #include "core/layout/TextAutosizer.h"
 #include "core/page/AutoscrollController.h"
 #include "core/page/ChromeClient.h"
 #include "core/page/ContextMenuController.h"
 #include "core/page/DragController.h"
 #include "core/page/FocusController.h"
 #include "core/page/PointerLockController.h"
 #include "core/page/ScopedPageSuspender.h"
 #include "core/page/ValidationMessageClient.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
@@ skipping 102 matching lines @@
              : ViewportDescription();
 }
 
 ScrollingCoordinator* Page::scrollingCoordinator() {
   if (!m_scrollingCoordinator && m_settings->acceleratedCompositingEnabled())
     m_scrollingCoordinator = ScrollingCoordinator::create(this);
 
   return m_scrollingCoordinator.get();
 }
 
-String Page::mainThreadScrollingReasonsAsText() {
-  if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
-    return scrollingCoordinator->mainThreadScrollingReasonsAsText();
+String Page::mainThreadScrollingReasonsAsText(Frame* frame) {
+  if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    // TODO(pdr): This walk should be unified with ScrollingCoordinator's

[pdr., 2016/12/07 22:25:29]

Can you do this TODO in this patch? Basically, pull the frame iteration loop
(including that comment about OOPIF) into this function, and have this function
call the ScrollingCoordinator::mainThreadScrollingReasons(frame) for !SPV2, and
do the LayoutObject loop for SPV2.

+    // mainThreadScrollingReasons().
+    MainThreadScrollingReasons reasons = 0;
+    for (Frame* frame = mainFrame(); frame;
+         frame = frame->tree().traverseNext()) {
+      if (!frame->isLocalFrame())
+        continue;
+      // TODO(alexmos,kenrb): For OOPIF, local roots that are different from
+      // the main frame can't be used in the calculation, since they use
+      // different compositors with unrelated state, which breaks some of the
+      // calculations below.
+      if (toLocalFrame(frame)->localFrameRoot() != mainFrame())
+        continue;
+      if (const auto* frameView = toLocalFrame(frame)->view()) {
+        if (const auto* scrollNode = frameView->scroll())
+          reasons |= scrollNode->mainThreadScrollingReasons();
+        for (LayoutObject* object = frameView->layoutView(); object;
+             object = object->nextInPreOrder()) {
+          if (const auto* properties = object->paintProperties()) {
+            if (const auto* scrollNode = properties->scroll())
+              reasons |= scrollNode->mainThreadScrollingReasons();
+          }
+        }
+      }
+    }
+    return String(
+        MainThreadScrollingReason::mainThreadScrollingReasonsAsText(reasons)
+            .c_str());
+  } else {
+    if (auto* scrollingCoordinator = this->scrollingCoordinator())
+      return scrollingCoordinator->mainThreadScrollingReasonsAsText(frame);
+  }
 
   return String();
 }
 
 ClientRectList* Page::nonFastScrollableRects(const LocalFrame* frame) {
   if (ScrollingCoordinator* scrollingCoordinator =
           this->scrollingCoordinator()) {
     // Hits in compositing/iframes/iframe-composited-scrolling.html
     DisableCompositingQueryAsserts disabler;
     scrollingCoordinator->updateAfterCompositingChangeIfNeeded();
@@ skipping 359 matching lines @@
     : chromeClient(nullptr),
       contextMenuClient(nullptr),
       editorClient(nullptr),
       spellCheckerClient(nullptr) {}
 
 Page::PageClients::~PageClients() {}
 
 template class CORE_TEMPLATE_EXPORT Supplement<Page>;
 
 }  // namespace blink