Fix the bug that sticky element may not be correctly invalidated due to non-promotion

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
index 43d57ebe4b042d11783e96ac70281c34fc84ca55..d737e85d72e9a4f3f460ed79dc20466776b43824 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp
@@ -372,6 +372,18 @@ void CompositingRequirementsUpdater::UpdateRecursive(
   RecursionData child_recursion_data = current_recursion_data;
   child_recursion_data.subtree_is_compositing_ = false;
 
+  if (layer->SticksToScroller() &&
+      layer->AncestorOverflowLayer()->ScrollsOverflow() &&

[flackr, 2017/05/12 18:00:46]

This seems to be duplicating the promotion logic in
CompositingReasonFinder::NonStyleDeterminedDirectReasons, isn't the right change
to make sure that we pass ignore_lcd_text = true to NonStyleDeterminedReasons
when we have a composited ancestor?

[yigu, 2017/05/17 19:39:05]

Done.

+      current_recursion_data.compositing_ancestor_ &&
+      current_recursion_data.compositing_ancestor_->AncestorScrollingLayer() &&
+      // This is not exact and over-promotes sometimes if
+      // compositing_ancestor_ is not under the same scroller as the sticky
+      // element.
+      !current_recursion_data.compositing_ancestor_->AncestorScrollingLayer()

[flackr, 2017/05/12 18:00:46]

If we have a composited ancestor scrolling layer then ignore_lcd_text should be
true on line 279 leading to promotion of sticky elements.

[yigu, 2017/05/17 19:39:05]

It's possible that a sticky element have a composited ancestor but doesn't have
ancestor scrolling layer. To correct set ignore_lcd_text, I added another
variable
indicating that it has composited ancestor.

+           ->IsRootLayer()) {
+    reasons_to_composite |= kCompositingReasonScrollDependentPosition;
+  }
+
   bool will_be_composited_or_squashed =
       can_be_composited && RequiresCompositingOrSquashing(reasons_to_composite);
   if (will_be_composited_or_squashed) {