Add more specific metrics for main thread scrolling reasons

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
index 1ee5b2d22a5a7dec2d091f2c86020a1f9f2018db..264efedd96965b1cc55b09fe7dcc3920a6d73212 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -178,12 +178,39 @@ bool CompositingReasonFinder::requiresCompositingForAnimation(
   return style.shouldCompositeForCurrentAnimations();
 }
 
+// Check if current layer has layout object that contains properties that cause
+// main thread scrolling.
+void CompositingReasonFinder::updateMainThreadScrollingReasonDueToLayoutObject(
+    const PaintLayer* layer) const {
+  // TODO(yigu) Keeping the reasons in PaintLayerScrollableArea
+  // is not ideal. When a layout becomes to non-scrollable

[bokan, 2016/12/13 14:38:57]

nit: "layout becomes to" -> "layer becomes"

[bokan, 2016/12/13 14:38:57]

Also, perhaps counterintuitively, I believe all layers created for a LayoutBox
will have a PLSA -- even if they're not scrollable. Perhaps you just need to
make the PLSA decrement any counts it has in dispose()

[yigu, 2016/12/13 20:54:10]

Done.

[yigu, 2016/12/13 20:54:10]

Done.

+  // the reasons kept in that scrollable area cannot be updated
+  // in the frameView correctly.
+  // 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);
+    layer->layoutObject()
+        ->frameView()
+        ->adjustMainThreadReasonsDueToLayoutObject(
+            MainThreadScrollingReason::kHasOpacity,
+            layer->compositesWithOpacity());
+  }
+}
+
 bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(
     const PaintLayer* layer) const {
   if (layer->layoutObject()->style()->position() != FixedPosition &&
       layer->layoutObject()->style()->position() != StickyPosition)
     return false;
 
+  updateMainThreadScrollingReasonDueToLayoutObject(layer);

[bokan, 2016/12/13 14:38:57]

This is only meant for Fixed/Sticky layers? The name should make that clear.

[yigu, 2016/12/13 20:54:10]

No. It should be moved somewhere else.

+
   if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger) &&
       (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled() ||
        !layer->backgroundIsKnownToBeOpaqueInRect(