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

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

Index: third_party/WebKit/Source/core/page/Page.cpp
diff --git a/third_party/WebKit/Source/core/page/Page.cpp b/third_party/WebKit/Source/core/page/Page.cpp
index bcc4c7944f597920d7f7e1e3a8638cb9c095e1af..e8776d3b9dcabc2194e9b3176d6581e5f34f03af 100644
--- a/third_party/WebKit/Source/core/page/Page.cpp
+++ b/third_party/WebKit/Source/core/page/Page.cpp
@@ -42,6 +42,7 @@
 #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"
@@ -164,9 +165,40 @@ ScrollingCoordinator* Page::scrollingCoordinator() {
   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();
 }