Only store previous clip rects for PaintLayers that support subsequences.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PrePaintTreeWalk.h"
 
 #include "core/dom/DocumentLifecycle.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutMultiColumnSpannerPlaceholder.h"
@@ skipping 129 matching lines @@
       GeometryMapper::SourceToDestinationClipRect(local_state, ancestor_state);
   clip_rect.MoveBy(-FloatPoint(ancestor_paint_offset));
 }
 
 void PrePaintTreeWalk::InvalidatePaintLayerOptimizationsIfNeeded(
     const LayoutObject& object,
     PrePaintTreeWalkContext& context) {
   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) {
     context.ancestor_transformed_or_root_paint_layer = &paint_layer;
   }
 
   const auto& ancestor =
       context.ancestor_transformed_or_root_paint_layer->GetLayoutObject();
   PropertyTreeState ancestor_state = *ancestor.LocalBorderBoxProperties();
 
@@ skipping 141 matching lines @@
       }
       Walk(*ToFrameView(frame_view_base), context);
     }
     // TODO(pdr): Investigate RemoteFrameView (crbug.com/579281).
   }
 
   object.GetMutableForPainting().ClearPaintFlags();
 }
 
 }  // namespace blink