Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/paint/PaintChunker.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 "platform/graphics/paint/PaintChunker.h"
 
 #include "platform/RuntimeEnabledFeatures.h"
 
 namespace blink {
 
 #if DCHECK_IS_ON()
 static bool gNullPaintPropertyChecksDisabled = false;
 DisableNullPaintPropertyChecks::DisableNullPaintPropertyChecks()
     : m_disabler(&gNullPaintPropertyChecksDisabled, true) {}
 #endif
 
-PaintChunker::PaintChunker() {}
+PaintChunker::PaintChunker() : m_nextDisplayItemBehavior(DefaultBehavior) {}
 
 PaintChunker::~PaintChunker() {}
 
 void PaintChunker::updateCurrentPaintChunkProperties(
     const PaintChunk::Id* chunkId,
     const PaintChunkProperties& properties) {
   DCHECK(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
 
   m_currentChunkId = WTF::nullopt;
   if (chunkId)
     m_currentChunkId.emplace(*chunkId);
+
+  // TODO(chrishtr): why not just compare properties != m_currentProperties?
+  // It seems this would solve the "are the property tree states the same"?
+  if (properties.backfaceHidden != m_currentProperties.backfaceHidden ||
+      properties.willChangeTransform != m_currentProperties.willChangeTransform)
+    m_nextDisplayItemBehavior = RequiresSeparateChunk;
+
   m_currentProperties = properties;
 }
 
 bool PaintChunker::incrementDisplayItemIndex(const DisplayItem& item) {
   DCHECK(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
 
 #if DCHECK_IS_ON()
   if (!gNullPaintPropertyChecksDisabled) {
     // Property nodes should never be null because they should either be set to
     // properties created by a LayoutObject/FrameView, or be set to a non-null
     // root node. If these DCHECKs are hit we are missing a call to update the
     // properties. See: ScopedPaintChunkProperties.
     DCHECK(m_currentProperties.transform);
     DCHECK(m_currentProperties.clip);
     DCHECK(m_currentProperties.effect);
     DCHECK(m_currentProperties.scroll);
   }
 #endif
 
   ItemBehavior behavior;
   Optional<PaintChunk::Id> newChunkId;
-  if (DisplayItem::isForeignLayerType(item.getType())) {
+  if (m_nextDisplayItemBehavior == RequiresSeparateChunk) {
+    behavior = RequiresSeparateChunk;
+    if (!item.skippedCache() && m_currentChunkId)
+      newChunkId.emplace(*m_currentChunkId);
+  } else if (DisplayItem::isForeignLayerType(item.getType())) {
     behavior = RequiresSeparateChunk;
     // Use null chunkId if we are skipping cache, so that the chunk will not
     // match any old chunk and will be treated as brand new.
     if (!item.skippedCache())
       newChunkId.emplace(item.getId());
 
     // Clear m_currentChunkId so that any display items after the foreign layer
     // without a new chunk id will be treated as having no id to avoid the chunk
     // from using the same id as the chunk before the foreign layer chunk.
     m_currentChunkId = WTF::nullopt;
@@ skipping 52 matching lines @@
 Vector<PaintChunk> PaintChunker::releasePaintChunks() {
   Vector<PaintChunk> chunks;
   chunks.swap(m_chunks);
   m_chunkBehavior.clear();
   m_currentChunkId = WTF::nullopt;
   m_currentProperties = PaintChunkProperties();
   return chunks;
 }
 
 }  // namespace blink