Only store previous clip rects for PaintLayers that support subsequences.

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 2736ebb03cb8a76dc7c9d136bdc40fd8f0963dc7..92a09d8bd466204ed1768d87c1fe0f3f63a12690 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -157,6 +157,7 @@ PaintLayer::PaintLayer(LayoutBoxModelObject& layout_object)
       has_descendant_with_clip_path_(false),
       has_non_isolated_descendant_with_blend_mode_(false),
       has_ancestor_with_clip_path_(false),
+      has_descendant_that_supports_subsequence_caching_(false),
       self_painting_status_changed_(false),
       layout_object_(layout_object),
       parent_(0),
@@ -685,6 +686,7 @@ void PaintLayer::UpdateDescendantDependentFlags() {
     has_visible_descendant_ = false;
     has_non_isolated_descendant_with_blend_mode_ = false;
     has_descendant_with_clip_path_ = false;
+    has_descendant_that_supports_subsequence_caching_ = false;
 
     for (PaintLayer* child = FirstChild(); child;
          child = child->NextSibling()) {
@@ -700,6 +702,10 @@ void PaintLayer::UpdateDescendantDependentFlags() {
 
       has_descendant_with_clip_path_ |= child->HasDescendantWithClipPath() ||
                                         child->GetLayoutObject().HasClipPath();
+
+      has_descendant_that_supports_subsequence_caching_ |=
+          child->HasDescendantThatSupportsSubsequenceCaching() ||
+          child->SupportsSubsequenceCaching();
     }
 
     if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled() &&
@@ -2748,6 +2754,26 @@ bool PaintLayer::PaintsWithTransform(
           GetCompositingState() != kPaintsIntoOwnBacking);
 }
 
+bool PaintLayer::SupportsSubsequenceCaching() const {
+  // SVG paints atomically.
+  if (GetLayoutObject().IsSVGRoot())
+    return true;
+
+  // Create subsequence for only stacking contexts whose painting are atomic.
+  if (!StackingNode()->IsStackingContext())
+    return false;
+
+  // 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;
+
+  DCHECK(!Parent() || Parent()->HasDescendantThatSupportsSubsequenceCaching());
+
+  return true;
+}
+
 ScrollingCoordinator* PaintLayer::GetScrollingCoordinator() {
   Page* page = GetLayoutObject().GetFrame()->GetPage();
   return (!page) ? nullptr : page->GetScrollingCoordinator();