Avoid re-computing rel-pos offset during paint.

This is an expensive operation. See LayoutBoxModelObject::RelativePositionOffset.

Bug: 984965

Change-Id: I487d2eaa96ee63c8cff88d12a062feec53ddb05a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726896Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682944}

third_party/blink/renderer/core/paint/paint_layer.cc

@@ -328,9 +328,7 @@ void PaintLayer::UpdateLayerPositionsAfterLayout() {
 
 void PaintLayer::UpdateLayerPositionRecursive() {
   auto old_location = location_without_position_offset_;
-  auto old_offset_for_in_flow_rel_position =
-      rare_data_ ? rare_data_->offset_for_in_flow_rel_position
-                 : PhysicalOffset();
+  auto old_offset_for_in_flow_rel_position = OffsetForInFlowRelPosition();
   UpdateLayerPosition();
 
   if (location_without_position_offset_ != old_location) {
@@ -1525,9 +1523,13 @@ static inline const PaintLayer* AccumulateOffsetTowardsAncestor(
     return nullptr;
 
   location += layer->LocationWithoutPositionOffset();
-  if (layer->GetLayoutObject().IsInFlowPositioned())
+  if (layer->GetLayoutObject().IsRelPositioned()) {
+    location += layer->OffsetForInFlowRelPosition();
+  } else if (layer->GetLayoutObject().IsInFlowPositioned()) {
     location += layer->GetLayoutObject().OffsetForInFlowPosition();
+  }
   location -= PhysicalOffset(containing_layer->ScrolledContentOffset());
+
   return containing_layer;
 }
 

third_party/blink/renderer/core/paint/paint_layer.h

@@ -1112,6 +1112,11 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
     return needs_visual_overflow_recalc_;
   }
 
+  PhysicalOffset OffsetForInFlowRelPosition() const {
+    return rare_data_ ? rare_data_->offset_for_in_flow_rel_position
+                      : PhysicalOffset();
+  }
+
  private:
   void SetNeedsCompositingInputsUpdateInternal();