Add more specific metrics for main thread scrolling reasons

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 c9aff864c2fc5ab2aef09eda942e392dca6a0fdc..1cf1c7abed8c26c9edafce19f621532661daf418 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -94,7 +94,10 @@ ScrollingCoordinator::ScrollingCoordinator(Page* page)
       m_touchEventTargetRectsAreDirty(false),
       m_shouldScrollOnMainThreadDirty(false),
       m_wasFrameScrollable(false),
-      m_lastMainThreadScrollingReasons(0) {}
+      m_lastMainThreadScrollingReasons(0),
+      m_mainThreadScrollingReasonsCounter(
+          MainThreadScrollingReason::kMainThreadScrollingReasonCount,
+          0) {}
 
 ScrollingCoordinator::~ScrollingCoordinator() {
   DCHECK(!m_page);
@@ -289,6 +292,18 @@ void ScrollingCoordinator::updateLayerPositionConstraint(PaintLayer* layer) {
 
 void ScrollingCoordinator::willDestroyScrollableArea(
     ScrollableArea* scrollableArea) {
+  // Decrese the number of layer that has any main thread
+  // scrolling reasons stored in ScrollingCoordinator
+  if (scrollableArea->isPaintLayerScrollableArea()) {
+    for (uint32_t i = 0;
+         i < MainThreadScrollingReason::kMainThreadScrollingReasonCount; ++i) {
+      if (toPaintLayerScrollableArea(scrollableArea)
+              ->hasMainThreadScrollingReason(1 << i)) {
+        adjustMainThreadReasonsFromStyle(1 << i, false);
+      }
+    }
+  }
+
   removeWebScrollbarLayer(scrollableArea, HorizontalScrollbar);
   removeWebScrollbarLayer(scrollableArea, VerticalScrollbar);
 }
@@ -1164,6 +1179,9 @@ MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons()
     if (frameView->hasBackgroundAttachmentFixedObjects())
       reasons |=
           MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects;
+
+    reasons |= getMainThreadScrollingReasonsFromStyle();
+
     FrameView::ScrollingReasons scrollingReasons =
         frameView->getScrollingReasons();
     const bool mayBeScrolledByInput =
@@ -1217,4 +1235,26 @@ bool ScrollingCoordinator::frameViewIsDirty() const {
   return false;
 }
 
+void ScrollingCoordinator::adjustMainThreadReasonsFromStyle(uint32_t reason,
+                                                            bool increase) {
+  int index = MainThreadScrollingReason::getReasonIndex(reason);
+  DCHECK_GE(index, 0);
+  m_mainThreadScrollingReasonsCounter[index] += increase ? 1 : -1;
+  DCHECK_GE(m_mainThreadScrollingReasonsCounter[index], 0);
+}
+
+MainThreadScrollingReasons
+ScrollingCoordinator::getMainThreadScrollingReasonsFromStyle() const {
+  MainThreadScrollingReasons reasons =
+      static_cast<MainThreadScrollingReasons>(0);
+  for (uint32_t reason = 1;
+       reason < MainThreadScrollingReason::kMainThreadScrollingReasonCount;
+       ++reason) {
+    if (m_mainThreadScrollingReasonsCounter[reason] > 0) {
+      reasons |= 1 << (reason - 1);
+    }
+  }
+  return reasons;
+}
+
 }  // namespace blink