Handle nested position:sticky elements correctly (compositor)

cc/trees/property_tree_builder.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/property_tree_builder.h"
 
 #include <stddef.h>
 
 #include <map>
 #include <set>
@@ skipping 741 matching lines @@
       // viewport bounds change, but do not unconditionally move by that delta
       // like fixed position nodes.
       if (scroll_ancestor->scrolls_inner_viewport) {
         data_for_children->property_trees->transform_tree
             .AddNodeAffectedByInnerViewportBoundsDelta(node->id);
       } else if (scroll_ancestor->scrolls_outer_viewport) {
         data_for_children->property_trees->transform_tree
             .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;
   data_from_ancestor.property_trees->transform_tree.UpdateTransforms(node->id);
 
   layer->set_offset_to_transform_parent(gfx::Vector2dF());
 
   // Flattening (if needed) will be handled by |node|.
   layer->set_should_flatten_transform_from_property_tree(false);
 
@@ skipping 771 matching lines @@
       root_layer, page_scale_layer, inner_viewport_scroll_layer,
       outer_viewport_scroll_layer, overscroll_elasticity_layer,
       elastic_overscroll, page_scale_factor, device_scale_factor, viewport,
       device_transform, property_trees, color);
   property_trees->effect_tree.CreateOrReuseRenderSurfaces(
       &render_surfaces, root_layer->layer_tree_impl());
   property_trees->ResetCachedData();
 }
 
 }  // namespace cc