Skip paint chunks with effectively invisible opacity.

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 2748 matching lines @@
 
   // The layer doesn't have children. Subsequence caching is not worth it,
   // because normally the actual painting will be cheap.
   // SVG is also painted atomically.
   if (!PaintLayerStackingNodeIterator(*StackingNode(), kAllChildren).Next())
     return false;
 
   return true;
 }
 
+bool PaintLayer::IsComposited() const {

[wkorman, 2017/04/20 22:23:56]

There is other logic in PaintLayer that checks GetCompositingState() and that
perhaps we'd want to follow this method instead, but I thought best to keep this
change focused on this one spot.

I also considered adding a HasDirectCompositingReasons() method to
ObjectPaintProperties to encapsulate the below but OPP is a very simple data
model currently and so I hesitated to add such logic there for currently just
this one call site, but open to thought.

+  if (RuntimeEnabledFeatures::slimmingPaintV2Enabled()) {
+    // In SPv2 paint layers always have a compositing state of
+    // kNotComposited. We instead determine compositing state by checking for a
+    // direct compositing reason on either the transform or effect node.
+    const ObjectPaintProperties* properties =
+        GetLayoutObject().PaintProperties();
+    if (!properties)
+      return false;
+    const TransformPaintPropertyNode* transform = properties->Transform();
+    if (transform && transform->HasDirectCompositingReasons())
+      return true;
+    const EffectPaintPropertyNode* effect = properties->Effect();
+    if (effect && effect->HasDirectCompositingReasons())
+      return true;
+    return false;
+  }
+  return GetCompositingState() == kPaintsIntoOwnBacking;
+}
+
+bool PaintLayer::PaintsWithTransparency(
+    GlobalPaintFlags global_paint_flags) const {
+  return IsTransparent() &&
+         ((global_paint_flags & kGlobalPaintFlattenCompositingLayers) ||
+          !IsComposited());
+}
+
 ScrollingCoordinator* PaintLayer::GetScrollingCoordinator() {
   Page* page = GetLayoutObject().GetFrame()->GetPage();
   return (!page) ? nullptr : page->GetScrollingCoordinator();
 }
 
 bool PaintLayer::CompositesWithTransform() const {
   return TransformAncestor() || Transform();
 }
 
 bool PaintLayer::CompositesWithOpacity() const {
@@ skipping 522 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif