Only store previous clip rects for PaintLayers that support subsequences.

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

Index: third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
diff --git a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
index 53ebcc64224eea42f9f30b9f3b5b941acca85d57..0032a8f3e66e0f5fc19a7b5826d4dada0b36f70e 100644
--- a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
+++ b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
@@ -147,7 +147,19 @@ void PrePaintTreeWalk::InvalidatePaintLayerOptimizationsIfNeeded(
   if (!object.HasLayer())
     return;
 
+  // This method checks whether any changes to descendant-affecting clips
+  // apply to any PaintLayer that supports subsequence caching. The
+  // only PaintLayers that can have such an effect by themselves (i.e.
+  // without any ancestor changes) are PaintLayers that have clip-related
+  // properties and descendants (including self) that support subsequence
+  // caching.
   PaintLayer& paint_layer = *ToLayoutBoxModelObject(object).Layer();
+  if (!paint_layer.SupportsSubsequenceCaching() &&
+      (RuntimeEnabledFeatures::slimmingPaintV2Enabled() ||

[chrishtr, 2017/04/13 17:19:54]

I also added this for symmetry with PaintInvalidator.

+       !paint_layer.HasDescendantThatSupportsSubsequenceCaching() ||
+       !paint_layer.GetLayoutObject().HasClipRelatedProperty()))
+    return;
+
   if (object.StyleRef().HasTransform() ||
       &object ==
           context.paint_invalidator_context.paint_invalidation_container) {