Don't allow the composited-alpha folding optimization to cross subsequences

third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp

Index: third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
index 0b017b04664f39301d876849fb16377bc756bc75..1b3e6ca9a1977dc8d1e33aebd97185f15517d935 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PaintController.cpp
@@ -163,6 +163,7 @@ void PaintController::AddCachedSubsequence(const DisplayItemClient& client,
          new_cached_subsequences_.end());
 
   new_cached_subsequences_.insert(&client, SubsequenceMarkers(start, end));
+  last_cached_subsequence_end_ = end;
 }
 
 bool PaintController::LastDisplayItemIsNoopBegin() const {
@@ -173,6 +174,10 @@ bool PaintController::LastDisplayItemIsNoopBegin() const {
   return last_display_item.IsBegin() && !last_display_item.DrawsContent();
 }
 
+bool PaintController::LastDisplayItemIsSubsequenceEnd() const {
+  return last_cached_subsequence_end_ == new_display_item_list_.size() - 1;

[wkorman, 2017/04/11 20:52:25]

We initialize last_cached_subsequence_end_ to 0, and so I wonder whether if we
call this with a display item list of size 1, but where we never actually called
AddCachedSubsequence, we might return true here when in fact we shouldn't.
Possible? Unit tests to explore could be helpful.

[chrishtr, 2017/04/11 21:09:10]

Fixed.

+}
+
 void PaintController::RemoveLastDisplayItem() {
   if (new_display_item_list_.IsEmpty())
     return;
@@ -536,6 +541,7 @@ void PaintController::CommitNewDisplayItems(
 
   new_cached_subsequences_.Swap(current_cached_subsequences_);
   new_cached_subsequences_.Clear();
+  last_cached_subsequence_end_ = 0;
   for (auto& item : current_cached_subsequences_) {
     item.key->SetDisplayItemsCached(current_cache_generation_);
 #if CHECK_DISPLAY_ITEM_CLIENT_ALIVENESS