Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index 762ab7bff80d818af0700a35f89fe35f65b5c10f..ae46639e44795182fd95910819f543f6bf941761 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -28,6 +28,8 @@
 
 #include "core/layout/LayoutView.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
+#include "core/page/Page.h"
+#include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/PaintLayerStackingNode.h"
 #include "core/paint/PaintLayerStackingNodeIterator.h"
@@ -250,6 +252,8 @@ void CompositingRequirementsUpdater::updateRecursive(
   CompositingReasons directReasons =
       m_compositingReasonFinder.directReasons(layer);
 
+  updateMainThreadScrollingReasonsFromStyle(layer);
+
   // Video is special. It's the only PaintLayer type that can both have
   // PaintLayer children and whose children can't use its backing to render
   // into. These children (the controls) always need to be promoted into their
@@ -544,4 +548,27 @@ void CompositingRequirementsUpdater::updateRecursive(
   layer->setCompositingReasons(reasonsToComposite);
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void CompositingRequirementsUpdater::updateMainThreadScrollingReasonsFromStyle(
+    const PaintLayer* layer) const {
+  // If current layer has opacity and has not been recorded, add the layer into
+  // frameView and vice versa.
+  if (!layer || !layer->getScrollableArea())
+    return;
+  if (layer->compositesWithOpacity() !=
+      layer->getScrollableArea()->hasMainThreadScrollingReason(
+          MainThreadScrollingReason::kHasOpacity)) {
+    layer->getScrollableArea()->flipMainThreadScrollingReason(
+        MainThreadScrollingReason::kHasOpacity);
+
+    if (ScrollingCoordinator* scrollingCoordinator =

[bokan, 2016/12/15 20:47:45]

Should we early out above if there's no scrolling coordinator? Seems like we
shouldn't set the scrolling reason on the PaintLayer unless we do it on the
ScrollingCoordinator too (otherwise if we add one after they'll be out of sync)

[yigu, 2016/12/16 18:17:39]

Done.

+            m_layoutView.document().page()->scrollingCoordinator()) {
+      scrollingCoordinator->adjustMainThreadReasonsFromStyle(
+          MainThreadScrollingReason::kHasOpacity,
+          layer->compositesWithOpacity());
+    }
+  }
+}
+
 }  // namespace blink