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

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 6fde443334dd11e05976d3406fb927cb3e945a76..02cd8a75fee2a5c031052d5c619e239c5bf94d55 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -190,7 +190,12 @@ void ScrollingCoordinator::updateAfterCompositingChangeIfNeeded() {
   if (m_shouldScrollOnMainThreadDirty ||
       m_wasFrameScrollable != frameIsScrollable) {
     setShouldUpdateScrollLayerPositionOnMainThread(
-        mainThreadScrollingReasons());
+        mainThreadScrollingReasons(m_page->deprecatedLocalMainFrame()));
+
+    // Need to update scroll on main thread reasons for subframe because
+    // subframe (e.g. iframe with background-attachment:fixed) should
+    // scroll on main thread while the main frame scrolls on impl.
+    updateSubFrameScrollOnMainReason();
     m_shouldScrollOnMainThreadDirty = false;
   }
   m_wasFrameScrollable = frameIsScrollable;
@@ -832,6 +837,26 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(
   }
 }
 
+void ScrollingCoordinator::updateSubFrameScrollOnMainReason() {
+  MainThreadScrollingReasons reasons =
+      static_cast<MainThreadScrollingReasons>(0);
+  for (Frame* frame = m_page->mainFrame(); frame;
+       frame = frame->tree().traverseNext()) {
+    if (!frame->isLocalFrame())
+      continue;
+    if (toLocalFrame(frame)->localFrameRoot() != m_page->mainFrame())
+      continue;
+
+    reasons = mainThreadScrollingReasons(toLocalFrame(frame));
+    if (reasons) {
+      if (WebLayer* scrollLayer =
+              toWebLayer(toLocalFrame(frame)->view()->layerForScrolling())) {
+        scrollLayer->addMainThreadScrollingReasons(reasons);
+      }
+    }

[flackr, 2016/12/07 21:53:11]

Do the reasons need to be cleared if we no longer have main thread scrolling
reasons? Can you add a test which verifies that the main thread scrolling
reasons go away (on the same frame, and on a child frame) if the cause goes
away?

[yigu, 2016/12/08 18:52:29]

Done.

+  }
+}
+
 void ScrollingCoordinator::layerTreeViewInitialized(
     WebLayerTreeView& layerTreeView) {
   if (Platform::current()->isThreadedAnimationEnabled()) {
@@ -1138,8 +1163,8 @@ bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(
   return false;
 }
 
-MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons()
-    const {
+MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons(
+    LocalFrame* targetFrame) const {
   MainThreadScrollingReasons reasons =
       static_cast<MainThreadScrollingReasons>(0);
 
@@ -1149,22 +1174,22 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons()
   if (!m_page->mainFrame()->isLocalFrame())
     return reasons;
 
-  // TODO(flackr) Currently we combine reasons for main thread scrolling from
-  // all frames but we should only look at the targetted frame (and its
-  // ancestors if the scroll bubbles up). http://crbug.com/568901
-  for (Frame* frame = m_page->mainFrame(); frame;
-       frame = frame->tree().traverseNext()) {
-    if (!frame->isLocalFrame())
+  // Walk through the subtree from the target frame to root. Use the gathered
+  // reasons to determine whether the target frame should be scrolled on main
+  // thread regardless other subframes on the same page.
+  for (Frame* checkFrame = targetFrame; checkFrame;

[flackr, 2016/12/07 21:53:11]

nit: This can continue to just be called frame.

[yigu, 2016/12/08 18:52:29]

Done.

+       checkFrame = checkFrame->tree().parent()) {
+    if (!checkFrame->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() != m_page->mainFrame())
+    if (toLocalFrame(checkFrame)->localFrameRoot() != m_page->mainFrame())
       continue;
 
-    FrameView* frameView = toLocalFrame(frame)->view();
+    FrameView* frameView = toLocalFrame(checkFrame)->view();
     if (!frameView || frameView->shouldThrottleRendering())
       continue;
 
@@ -1193,11 +1218,19 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons()
   return reasons;
 }
 
-String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const {
-  DCHECK(m_page->deprecatedLocalMainFrame()->document()->lifecycle().state() >=
+String ScrollingCoordinator::mainThreadScrollingReasonsAsText(
+    Frame* frame) const {
+  DCHECK(toLocalFrame(frame)->document()->lifecycle().state() >=
          DocumentLifecycle::CompositingClean);
-  if (WebLayer* scrollLayer = toWebLayer(
-          m_page->deprecatedLocalMainFrame()->view()->layerForScrolling())) {
+
+  // If an iframe has attributes like background-attachment: fixed
+  // it should scroll on main thread. The reasons is added in
+  // mainThreadScrollingReasons. Need to call this function
+  // before access such reason.
+  mainThreadScrollingReasons(toLocalFrame(frame));

[flackr, 2016/12/07 21:53:11]

This shouldn't be necessary anymore right?

[yigu, 2016/12/08 18:52:29]

Done.

+
+  if (WebLayer* scrollLayer =
+          toWebLayer(toLocalFrame(frame)->view()->layerForScrolling())) {
     String result(MainThreadScrollingReason::mainThreadScrollingReasonsAsText(
                       scrollLayer->mainThreadScrollingReasons())
                       .c_str());