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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 823 matching lines @@
               .OffsetForInFlowPositionedInline(ToLayoutBox(GetLayoutObject()));
       local_point += offset;
     }
   }
 
   if (GetLayoutObject().IsInFlowPositioned()) {
     LayoutSize new_offset = GetLayoutObject().OffsetForInFlowPosition();
     if (rare_data_ || !new_offset.IsZero())
       EnsureRareData().offset_for_in_flow_position = new_offset;
     local_point.Move(new_offset);
+    if (GetLayoutObject().Style()->GetPosition() == EPosition::kSticky &&
+        !new_offset.IsZero()) {

[flackr, 2017/04/26 15:21:01]

This will invalidate as long as the offset is not zero, but we only need to
invalidate if the offset changed. I think you should add some invalidation tests
that show that the sticky position element doesn't invalidate if it hasn't moved
(i.e. hasn't passed the starting constraint yet). See for example
https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/paint/inv...

[yigu, 2017/04/26 21:46:57]

Done.

+      ObjectPaintInvalidator(GetLayoutObject())
+          .InvalidatePaintIncludingNonCompositingDescendants();
+    }
   } else if (rare_data_) {
     rare_data_->offset_for_in_flow_position = LayoutSize();
   }
 
   location_ = local_point;
 
   if (scrollable_area_ && did_resize)
     scrollable_area_->VisibleSizeChanged();
 
 #if DCHECK_IS_ON()
@@ skipping 2462 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif