Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index ac22ac5ab04f247e1d77926b9acabb6a835c8cac..e2090aaef65d5c21b8317e05ede26ebb29611c70 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -106,7 +106,8 @@ PaintLayerScrollableArea::PaintLayerScrollableArea(PaintLayer& layer)
       m_scrollbarManager(*this),
       m_scrollCorner(nullptr),
       m_resizer(nullptr),
-      m_scrollAnchor(this)
+      m_scrollAnchor(this),
+      m_reasons(0)
 #if DCHECK_IS_ON()
       ,
       m_hasBeenDisposed(false)
@@ -1750,10 +1751,37 @@ static bool layerNeedsCompositedScrolling(
            layer->layoutObject()->style()->hasBorderRadius());
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void PaintLayerScrollableArea::updateMainThreadScrollingReasonsFromStyle(

[bokan, 2016/12/19 14:54:25]

Sorry to waffle on the name again, but after seeing it a bunch of times I think
updateStyleRelatedMainThreadScrollingReasons feels less awkward.

[yigu, 2016/12/19 18:55:04]

Done.

+    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())

[bokan, 2016/12/19 14:54:25]

Now that we're in PLSA you don't have to check for layer or scrollable area (you
can DCHECK(layer()))

[yigu, 2016/12/19 18:55:04]

Done.

+    return;
+
+  if (!box().frame()->page()->scrollingCoordinator())
+    return;
+  ScrollingCoordinator* scrollingCoordinator =
+      box().frame()->page()->scrollingCoordinator();
+  if (layer->compositesWithOpacity() !=
+      layer->getScrollableArea()->hasMainThreadScrollingReason(

[bokan, 2016/12/19 14:54:25]

layer->getScrollableArea() isn't needed anymore.

[yigu, 2016/12/19 18:55:04]

Done.

+          MainThreadScrollingReason::kHasOpacity)) {
+    layer->getScrollableArea()->flipMainThreadScrollingReason(
+        MainThreadScrollingReason::kHasOpacity);
+
+    scrollingCoordinator->adjustMainThreadReasonsFromStyle(
+        MainThreadScrollingReason::kHasOpacity, layer->compositesWithOpacity());
+  }
+}
+
 void PaintLayerScrollableArea::updateNeedsCompositedScrolling(
     LCDTextMode mode) {
   const bool needsCompositedScrolling =
       layerNeedsCompositedScrolling(mode, layer());
+  if (!needsCompositedScrolling)
+    updateMainThreadScrollingReasonsFromStyle(layer());
+
   if (static_cast<bool>(m_needsCompositedScrolling) !=
       needsCompositedScrolling) {
     m_needsCompositedScrolling = needsCompositedScrolling;