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

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

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 355 matching lines @@
   }
 
   reasons_to_composite |= overlap_compositing_reason;
 
   // The children of this layer don't need to composite, unless there is
   // a compositing layer among them, so start by inheriting the compositing
   // ancestor with m_subtreeIsCompositing set to false.
   RecursionData child_recursion_data = current_recursion_data;
   child_recursion_data.subtree_is_compositing_ = false;
 
+  if (layer->SticksToScroller() &&
+      layer->AncestorOverflowLayer()->ScrollsOverflow() &&
+      current_recursion_data.compositing_ancestor_ &&
+      current_recursion_data.compositing_ancestor_->AncestorScrollingLayer() &&

[chrishtr, 2017/05/10 19:29:36]

Please add the comments explaining that this is not exact and over-promotes
sometimes
if compositing_ancestor_ is not under the same scroller as the sticky element.

[yigu, 2017/05/10 20:01:42]

Done.

+      !current_recursion_data.compositing_ancestor_->AncestorScrollingLayer()
+           ->IsRootLayer()) {
+    reasons_to_composite |= kCompositingReasonScrollDependentPosition;
+  }
+
   bool will_be_composited_or_squashed =
       can_be_composited && RequiresCompositingOrSquashing(reasons_to_composite);
   if (will_be_composited_or_squashed) {
     // This layer now acts as the ancestor for kids.
     child_recursion_data.compositing_ancestor_ = layer;
 
     // Here we know that all children and the layer's own contents can blindly
     // paint into this layer's backing, until a descendant is composited. So, we
     // don't need to check for overlap with anything behind this layer.
     overlap_map.BeginNewOverlapTestingContext();
@@ skipping 171 matching lines @@
     descendant_has3d_transform |=
         any_descendant_has3d_transform || layer->Has3DTransform();
   }
 
   // At this point we have finished collecting all reasons to composite this
   // layer.
   layer->SetCompositingReasons(reasons_to_composite);
 }
 
 }  // namespace blink