Skip paint chunks with effectively invisible opacity.

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
index 83cf5e45e90c8d5f3da455493a94a93311c6a8d0..29560ad32793a176ffcc736f27223a5f24415299 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -499,12 +499,28 @@ void PaintArtifactCompositor::LayerizeGroup(
   // due to copying the chunk list. Subtotal: O((qd + p)d) = O(qd^2 + pd)
   // Assuming p > d, the total complexity would be O(pqd + qd^2 + pd) = O(pqd)
   while (chunk_it != paint_artifact.PaintChunks().end()) {
-    // Look at the effect node of the next chunk. There are 3 possible cases:
+    // Look at the effect node of the next chunk. If the chunk has
+    // effectively-invisible opacity we skip it. Otherwise, there are 3 possible
+    // cases:
     // A. The next chunk belongs to the current group but no subgroup.
     // B. The next chunk does not belong to the current group.
     // C. The next chunk belongs to some subgroup of the current group.
     const EffectPaintPropertyNode* chunk_effect =
         chunk_it->properties.property_tree_state.Effect();
+
+    // Skip paint chunks that are effectively invisible due to opacity as long
+    // as we don't have another explicit reason to give them a layer. The lower
+    // bound of visibility is considered to be 0.0004f < 1/2048. With 10-bit
+    // color channels (only available on the newest Macs as of 2016; otherwise
+    // it's 8-bit), we see that an alpha of 1/2048 or less leads to a color
+    // output of less than 0.5 in all channels, hence not visible.
+    static const float kMinimumVisibleOpacity = 0.0004f;
+    if (chunk_effect->Opacity() < kMinimumVisibleOpacity &&
+        !chunk_effect->HasDirectCompositingReasons()) {
+      chunk_it++;
+      continue;
+    }
+
     if (chunk_effect == &current_group) {
       // Case A: The next chunk belongs to the current group but no subgroup.
       bool is_foreign =