Handle nested position:sticky elements correctly (compositor)

cc/trees/property_tree_builder.cc

Index: cc/trees/property_tree_builder.cc
diff --git a/cc/trees/property_tree_builder.cc b/cc/trees/property_tree_builder.cc
index ef843d5d9890e67baba1396156fbca65bf06cb85..f0df2c19c19533560f66ec39b6040e21771aeac8 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -759,10 +759,32 @@ bool AddTransformNodeIfNeeded(
             .AddNodeAffectedByOuterViewportBoundsDelta(node->id);
       }
     }
+    // This calculation will be incorrect if we have an ancestor sticky element
+    // that is not composited: http://crbug.com/702229
     sticky_data->main_thread_offset =
         layer->position().OffsetFromOrigin() -
         sticky_data->constraints.parent_relative_sticky_box_offset
             .OffsetFromOrigin();
+
+    // Copy the ancestor nodes for later use. These layers are guaranteed to
+    // have transform nodes at this point because they are our ancestors (so
+    // have already been processed) and are sticky (so have transform nodes).
+    int x = sticky_data->constraints.nearest_layer_shifting_sticky_box;

[flackr, 2017/03/17 18:04:08]

nit: Use descriptive variable names

[smcgruer, 2017/03/17 18:42:42]

Done.

+    if (x >= 0) {

[flackr, 2017/03/17 18:04:08]

Compare to the invalid id constant.

[smcgruer, 2017/03/17 18:42:42]

Done.

+      DCHECK(data_for_children->property_trees->IsInIdToIndexMap(
+          PropertyTrees::TreeType::TRANSFORM, x));
+      sticky_data->nearest_node_shifting_sticky_box =
+          data_for_children->property_trees
+              ->layer_id_to_transform_node_index[x];
+    }
+    int y = sticky_data->constraints.nearest_layer_shifting_containing_block;
+    if (y >= 0) {
+      DCHECK(data_for_children->property_trees->IsInIdToIndexMap(
+          PropertyTrees::TreeType::TRANSFORM, y));
+      sticky_data->nearest_node_shifting_containing_block =
+          data_for_children->property_trees
+              ->layer_id_to_transform_node_index[y];
+    }
   }
 
   node->needs_local_transform_update = true;