Unify the calculation of main thread offset of sticky element

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 096496646d08d65e0cdb5cfd35bebedfa4fe21d3..94b8866c0d2c770b66973500df785246b0d1b869 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -282,8 +282,7 @@ void CompositedLayerMapping::UpdateBackdropFilters(const ComputedStyle& style) {
 }
 
 void CompositedLayerMapping::UpdateStickyConstraints(
-    const ComputedStyle& style,
-    const PaintLayer* compositing_container) {
+    const ComputedStyle& style) {
   bool sticky = style.GetPosition() == EPosition::kSticky;
   const PaintLayer* ancestor_overflow_layer =
       owning_layer_.AncestorOverflowLayer();
@@ -297,6 +296,7 @@ void CompositedLayerMapping::UpdateStickyConstraints(
   }
 
   WebLayerStickyPositionConstraint web_constraint;
+  WebSize sticky_main_thread_offset;

[smcgruer, 2017/05/26 17:14:12]

Nit; this method is explicitly about sticky, we could just name this
'main_thread_offset' within this method.

[yigu, 2017/05/26 18:04:21]

Done.

   if (sticky) {
     const StickyConstraintsMap& constraints_map =
         ancestor_overflow_layer->GetScrollableArea()->GetStickyConstraintsMap();
@@ -307,42 +307,8 @@ void CompositedLayerMapping::UpdateStickyConstraints(
     // composited layer. This information is used by the compositor side to
     // compute the additional offset required to keep the element stuck under
     // compositor scrolling.
-    //
-    // Starting from the scroll container relative location, removing the
-    // enclosing layer's offset and the content offset in the composited layer
-    // results in the parent-layer relative offset.
-    FloatPoint parent_relative_sticky_box_offset =
-        constraints.ScrollContainerRelativeStickyBoxRect().Location();
-
-    // The enclosing layers offset returned from |convertToLayerCoords| must be
-    // adjusted for both scroll and ancestor sticky elements.
-    LayoutPoint enclosing_layer_offset;
-    compositing_container->ConvertToLayerCoords(ancestor_overflow_layer,
-                                                enclosing_layer_offset);
-    DCHECK(!ScrollParent() || ScrollParent() == ancestor_overflow_layer);
-    if (!ScrollParent() && compositing_container != ancestor_overflow_layer) {
-      enclosing_layer_offset += LayoutSize(
-          ancestor_overflow_layer->GetScrollableArea()->GetScrollOffset());
-    }
-    // TODO(smcgruer): Until http://crbug.com/702229 is fixed, the nearest
-    // sticky ancestor may be non-composited which will make this offset wrong.
-    if (const LayoutBoxModelObject* ancestor =
-            constraints.NearestStickyAncestor()) {
-      enclosing_layer_offset -=
-          RoundedIntSize(constraints_map.at(ancestor->Layer())
-                             .GetTotalContainingBlockStickyOffset());
-    }
-
-    DCHECK(!content_offset_in_compositing_layer_dirty_);
-    parent_relative_sticky_box_offset.MoveBy(
-        FloatPoint(-enclosing_layer_offset) -
-        FloatSize(ContentOffsetInCompositingLayer()));
-
-    if (compositing_container != ancestor_overflow_layer) {
-      parent_relative_sticky_box_offset.MoveBy(
-          FloatPoint(compositing_container->GetCompositedLayerMapping()
-                         ->ContentOffsetInCompositingLayer()));
-    }
+    sticky_main_thread_offset =
+        RoundedIntSize(GetLayoutObject().StickyPositionOffset());
 
     web_constraint.is_sticky = true;
     web_constraint.is_anchored_left =
@@ -361,8 +327,6 @@ void CompositedLayerMapping::UpdateStickyConstraints(
     web_constraint.right_offset = constraints.RightOffset();
     web_constraint.top_offset = constraints.TopOffset();
     web_constraint.bottom_offset = constraints.BottomOffset();
-    web_constraint.parent_relative_sticky_box_offset =
-        RoundedIntPoint(parent_relative_sticky_box_offset);
     web_constraint.scroll_container_relative_sticky_box_rect =
         EnclosingIntRect(constraints.ScrollContainerRelativeStickyBoxRect());
     web_constraint.scroll_container_relative_containing_block_rect =
@@ -397,6 +361,7 @@ void CompositedLayerMapping::UpdateStickyConstraints(
   }
 
   graphics_layer_->SetStickyPositionConstraint(web_constraint);
+  graphics_layer_->SetStickyMainThreadOffset(sticky_main_thread_offset);
 }
 
 void CompositedLayerMapping::UpdateLayerBlendMode(const ComputedStyle& style) {
@@ -1113,7 +1078,7 @@ void CompositedLayerMapping::UpdateGraphicsLayerGeometry(
                                           graphics_layer_parent_location);
   UpdateContentsOffsetInCompositingLayer(
       snapped_offset_from_composited_ancestor, graphics_layer_parent_location);
-  UpdateStickyConstraints(GetLayoutObject().StyleRef(), compositing_container);
+  UpdateStickyConstraints(GetLayoutObject().StyleRef());
   UpdateSquashingLayerGeometry(
       graphics_layer_parent_location, compositing_container, squashed_layers_,
       squashing_layer_.get(),