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

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 16f625873668c322d216058212ebb91e6e3d0ab5..67cc67149558c7ce349d0749f3d422bbb651603e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -838,6 +838,11 @@ void PaintLayer::UpdateLayerPosition() {
 
   if (GetLayoutObject().IsInFlowPositioned()) {
     LayoutSize new_offset = GetLayoutObject().OffsetForInFlowPosition();
+    if (!HasCompositedLayerMapping() &&

[chrishtr, 2017/04/28 20:11:43]

This is not in general the right place to be doing paint invalidations like
this.
But maybe there is something new to add...could you explain the core issue
thatis happening in these cases? I'm not yet sure what it is.

Please also update the title of the patch to match the description of the core
issue.

[yigu, 2017/04/28 20:38:56]

Suppose we have a region A that needs repaint on scroll and it has a composited
but main thread scrolled child B. When we recursively do paint invalidation from
A we may stop at the composited child B so B's non-composited children cannot
get invalidated. Here we invalidate the children if their location changed.

+        OffsetForInFlowPosition() != new_offset) {
+      GetLayoutObject().SetShouldDoFullPaintInvalidation(
+          kPaintInvalidationLocationChange);
+    }
     if (rare_data_ || !new_offset.IsZero())
       EnsureRareData().offset_for_in_flow_position = new_offset;
     local_point.Move(new_offset);