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..e39f3567e2940520de168522de1c1bfe7bb45ad0 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 +252,9 @@ void CompositingRequirementsUpdater::updateRecursive(
   CompositingReasons directReasons =
       m_compositingReasonFinder.directReasons(layer);
 
+  if (!compositor->preferCompositingToLCDTextEnabled())
+    updateMainThreadScrollingReasonsFromStyle(layer);

[bokan, 2016/12/16 23:06:01]

Sorry, one last thing. I think this is the wrong place for this.
CompositingRequirementsUpdater is collecting reasons we *should* composite. This
looks like it rather belongs in PaintLayerScrollableArea's
layerNeedsCompositedScrolling, here particularily, where we return false:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/Pai...

[yigu, 2016/12/18 15:05:11]

Done.

+
   // 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 +549,28 @@ 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 (ScrollingCoordinator* scrollingCoordinator =

[bokan, 2016/12/16 23:06:01]

nit: we generally prefer the above pattern of early-return on null to prevent
excessive indenting.

[yigu, 2016/12/18 15:05:12]

Done.

+          m_layoutView.document().page()->scrollingCoordinator()) {
+    if (layer->compositesWithOpacity() !=
+        layer->getScrollableArea()->hasMainThreadScrollingReason(
+            MainThreadScrollingReason::kHasOpacity)) {
+      layer->getScrollableArea()->flipMainThreadScrollingReason(
+          MainThreadScrollingReason::kHasOpacity);
+
+      scrollingCoordinator->adjustMainThreadReasonsFromStyle(
+          MainThreadScrollingReason::kHasOpacity,
+          layer->compositesWithOpacity());
+    }
+  }
+}
+
 }  // namespace blink