Unify the calculation of main thread offset of sticky element

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 5d76895c46da812a4a79674bbfc21deb8e71769a..16daa7935f716147c8c63a7253c9b2c7d9f603f3 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -842,6 +842,17 @@ void PaintLayer::UpdateLayerPosition() {
     if (rare_data_ || !new_offset.IsZero())
       EnsureRareData().offset_for_in_flow_position = new_offset;
     local_point.Move(new_offset);
+
+    // Find the layout offset of the unshifted sticky box within its parent
+    // composited layer. This information is used by the compositor side to
+    // compute the additional offset required to keep the element stuck under
+    // compositor scrolling.
+    if (GetLayoutObject().Style()->GetPosition() == EPosition::kSticky &&
+        GetCompositedLayerMapping()) {
+      GetCompositedLayerMapping()
+          ->MainGraphicsLayer()
+          ->SetStickyMainThreadOffset(RoundedIntSize(new_offset));

[flackr, 2017/06/01 19:01:47]

I don't think this is the right place to set the sticky main thread offset on
the graphics layer since it may be created later. We should be doing this when
we set the graphics layer's position based, which is I believe in
CompositedLayerMapping::UpdateMainGraphicsLayerGeometry.

[yigu, 2017/06/01 19:28:40]

Done.

+    }
   } else if (rare_data_) {
     rare_data_->offset_for_in_flow_position = LayoutSize();
   }