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 1f9148f70c35bc50321b277d0a6b01410f2b6fb7..3cfc2669d76f980954bd6725e14221974db23df3 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -841,6 +841,11 @@ void PaintLayer::UpdateLayerPosition() {
     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();
   }